There are different strategies involved for 40 card decks versus 60 card decks.
A 60 card deck is constructed. This means you'll have access to a great number of dual lands and land fetches. A 40 card deck is limited. You will probably have only basic lands to work with, with may be one or two mana fixers you managed to pick up.
It's fairly easy to make the argument that the more cards in a color you have, the more you have to weigh into that color for your land base. That's very simple logic. In fact, it's almost too simple.
The first hole in that logic is that you'll have many more spells of one color in your deck than another. This really shouldn't be the case. If it was a sealed event, chances are your cards are fairly evenly balanced. You'll have 15-20 cards in each color, and more than likely you will play the two colors that both have 11 playables. You're unlikely to have a situation of 18 playables in one color and 5 in another color. If you had, let's just say, 18 red and 5 green, is it reasonable to think you had 5 or less playable cards in white, blue and black? No, it's not. Even if you did have 18 playable cards in red, you're probably still better off only playing the best 11 or 12 of those playables, and playing 9 or 10 white or black or blue or whatever color had the second most playables. If you were to bucket your cards into colors, and then sort each bucket by how playable the card is (not a scientific route, I recognize, but one that many players do) you will probably find that #'s 6-11 in one color are much better than 12-18th place in the main color. So your cards will be balanced anyway if you've built a good deck.
The second hole in that logic is that it doesn't take into account what this humble magic player finds to be the most important aspect of having multiple colored decks: color aligned cards. A color aligned card is one that has multiple colored mana symbols. Most cards in magic are in the form of {#}{x}
where # is a number and x is a color. In other words, they're like Runeclaw Bear which is 1G
and not like Garruk's Companion which is GG
. Garruk's Companion is color aligned to green. For it to be effective, you need to have your first two lands be forest. Playing mono green, that's easy. Playing any non-mono-green deck, that's hard. It gets even worse for cards like Leatherback Baloth and Phryexian Obliterator which are very powerful, but are very color aligned. Because most cards only require 1 mana in any particular color, you only need to worry about getting 1 of those lands casting the spell.
In the recent pre-release, I had two copies of Martial Law. Because I had a significant amount of control spells (Dispel Syncopate and friends) I felt the need to be able to cast multiple blue spells in a turn. For this reason, I had 9 islands, 6 plains, and 3 Azorius Guildgate. As it turned out, only once did I have the need to cast multiple counters, and at the same time, twice I had Martial Law and Sunspire Griffin in my hand but couldn't play them due to having only 1 source of white mana. If I could play it over again, I'd've gone 6 Islands, 9 plains to compliment my 3 gates. I had four cards in my deck that were turn 3 or turn 4 color aligned white, with no spells color aligned to blue (except for the 2WWUU
Sphinx, but since she's color aligned to both I didn't see it as a big issue. The likelihood of having 6 mana with only 1 source of either color was highly unlikely).
So here's what I do in limited:
- I have only one color that allows for color aligned cards. Exceptions to this are if I have an incredible amount of land fixing, or if there is a huge benefit to be gained (for example, Trostani, Selesnya's Voice or Jarad, Golgari Lich Lord. I would also consider making exceptions if I were in a situation where I pulled, for example, Murder and Deadbridge Goliath.) Remember how my pre-release didn't technically have blue-aligned, but I treated it as such because of how important I thought it was to cast two blue spells in a turn? (That's basically the same thing as having a blue aligned spell.) Trying to align to two colors was disastrous for me in that tournament. I went against a primary tenet of my deck building strategy, and I paid a hefty price (0-3 first three rounds, until I finally WTF'd.)
- Assuming I have no mana fixing, I will go with 9/8 basics for no color aligned cards. The 'winner' (the color with 9) is the one that has more cards in that color. I will go with 11/6 if I have 1 or 2 color aligned cards. I will go with 12/6 if I have three or more color aligned cards. Note that 12/6 adds to 18, not 17 as normal. Because there's so many color aligned spells, I can afford to play more lands. Why 'afford'? Because a color aligned spell in my deck will always be more powerful than a non color aligned spell of the same cost. If it wasn't significantly more powerful (compare Runeclaw Bear and Garruk's Companion again) I wouldn't align.
- If I have mana fixing that is either mana-free (like dual lands) or is very cheap (like Traveler's Amulet or Transguild Promenade) then it takes away from the biggest mana pool. For example, if I would have gone 12/6, and I get 3 fixers, it instead becomes 9/6 with 3 fixers.
- If I'm going 3 colors, I do not use color aligned cards. It becomes more difficult for me to set any hard and fast rules because I don't often go three colors in limited. This will probably change this block, since RTR is so color-happy, and perhaps I'll re-evaluate this answer after doing more RTR.
With constructed it's very different. I do not use formulas for constructed mana pool determination. I start with something that seems decent and I play test the deck against my other decks or against friends. With a constructed deck you have the luxury of playing the deck many, many times before you take it to a tournament. For this reason, you do not need to resort to using formulas which are always guesses to what your mana pool needs. The only way to really know if you have a good mana pool is to play test your deck many times. You don't time to do that in a limited environment, so you are forced to use a formula to approximate your ideal mana pool.
Also in constructed, you have access to a wide variety of dual lands and mana fixers. The key is to get the right mana at the right time. When you're play testing, keep track of how many times you want to play a particular card, but can't, and why you can't. Perhaps it's because you don't meet requirements of the card. This happened to me with metalcraft a lot in the Scars block: what was the point of playing a card whose benefit was only worth the mana if I had three artifacts out and I didn't have those out yet? That might mean I need to scrap the deck, or it might just mean I need to do some tweaking. If a card, game after game, can't come out on turn X where X is it's CMC, (and you want it to... you don't usually want to Murder on turn 3...), you either need to fix your mana base or scrap the card. Both are good or bad options depending on the situation, and you can't make hard rules about that. It's that decision making that separates good deckbuilders from great deckbuilders.