A Commander deck consists of 99 unique cards and 1 legendary creature. I'm still very unfamilier with this format. How would you go about figuring out the number of lands to put into your commander deck?
Because of the slower pace of play in EDH/Commander, it isn't as drastically important to hit 5-6 land drops in 5-6 turns. In addition, the number of "mana rocks" (mana-generating artifacts) and dual lands/mana filtering/land fetch cards are usually higher.
Considering that, the 40% lands rule is still fairly close to normal (lightened obviously for the reasons above). I've run 34-36 land decks successfully, but usually you see 37-40 with some of the previously mentioned accelerators.
Abnormal decks (with huge numbers of accelerators) will run as few as 32 lands (I've run 34 and 36 before with some success), but I'd recommend starting around 40 and cutting around one land per three accelerators (if you're trying to cut land, and not running very high-cost spells).
As an example, the Devour for Power Commander deck shipped with 40 lands and four mana-generating artifacts, but it is fairly heavy on costs.