Just to elaborate on ruds' excellent answer, there are 12% of seven card hands that are clearly "too good" to pre emmpt" (open with 1 of the suit instead with AKQxxxx), and 7% with Jxxxxxx or worse. which should be passed. Most people would not preempt with these 19% of hands in the "tails."
Most people would preempt with the 44% of hands in the upper middle that had two of the top three honors. That leaves 37% in the lower middle.
Of that 37% of suits, most people would pre empt with QJTxxxx, KJTxxxx or AJTxxxx. Most people would not pre empt with Q9xxxxx or weaker. That leaves QJxxxxx and QTxxxxx as a matter of choice among Q headed suits. My preference is for "light" preempts, and I would open these hands. Likewise, I would pass K7xxxxx and K8xxxxx, but would open the three K-headed hands between K9xxxxx and KJxxxxx. I would also open any hand with a suit headed by an ace, even A765432, because it has one sure top trick and four probable tricks on length if the suit becomes trump.
Adding "three of the top five" to the 44% gives you roughly half of the seven card suits being opened with a preempt by most people. Since I favor "light" preempts, partner will know that I will bid one of the suit with the strongest 12% of hands, pass with about the weakest 12% (after eliminating a few more of the weaker hands slightly stronger than Jxxxxxx), and open the three quarters in the middle unless we agree otherwise. In any event, the partnership discussion will be about these 37% of hands in the lower middle.
Median strengh is roughly hands with "two honors out of top three" with AQ being a bit above, KQ being a bit below, and KQT or AJT being right on. The lower limit of my range (QTxxxxx) is about a king weaker; a king stronger would put me at the top of my range at AKJTxxx. Other partnerships prefer to lower the "variance" to a queen or so.
The addition of side honors would reduce my tendency to preempt. For instance, I would preempt with KQxxxxx and six side suit spot cards, but not with KQxxxxx KQx xx x, or even KQxxxxx Kxxx x x, especially if the other king-headed suit were major. In the first case, I have a genuinely weak hand with 5 high card points; in the second, I have my "fair share" of 10 hcps, in the third, 8 hcps, close enough to average, so I don't feel the need to preempt.
If partner has nothing, the first hand has no defense against a small slam, and limited defense against a grand slam, which is why I am willing to risk a four trick set at any vulnerability except "unfavorable" (vulnerable vs not). In the second case, I should be able to defeat a grand slam, and possibly a small slam, on my own. So I favor a "light" preempt with 3-7 hcp (QTxxxxx barely counts as such, "adjusted") that is, with some offense and no defense.