Your response of "With 7 points?" makes me think you have some misconceptions about the hands the you ought to preempt on. (As an aside, I am really curious: What do you preempt on?).
One normally preempts on highly offensive hands, which when played in any suit other than your long suit will offer very little defense. The position also is relevant, as in third seat you can really be aggressive in your preempts, opposite a passed hand. The vulnerability and scoring also matters: at IMPS going down 3 doubled, non-vul (-500) against a making vul game is (-600) is a great sacrifice.
Don't look at the total hcp, but look at your suit length and how offensive your hand is.
So a hand like Jxxxxxx, Kx , Qx, Kx (9 hcp) is not a hand to prempt (it offers nice defense and is less offensive oriented), but you should preempt on a hand like KQTxxxx, x, xxx, xx (only 5 hcp).
With KQTxxxx as trumps, assuming partner holds 2 small trumps, you rate to take 5-6 tricks on average. If partner also has an outside A, you will take 6-7 tricks, which is a great bargain (going down 2 or 3) against opponents making game/slam, especially when vul.
The usual point of preempting is that you rate to take away opponent's bidding space, as well as pointing the way to partner in making a sacrifice over opponents game/slam bid.
Ultimately, like with any other bidding situation, it is a risk vs reward + partnership discipline scenario and if you don't go for a number sometimes, you are not preempting enough.