(If you don't have a calculator handy, use the tool below to do the calculations for you.)
If you've got a 10-point lead and the ball with 10 minutes left, is that a safe lead?
Of course not; teams come back from a 10-point deficit all the time. A 10-point lead, plus the ball, gives you a 7.5-point safety margin. It's safe for 56.25 seconds—56, rounded down. With 600 seconds to play, a 10-point lead (with the ball) is 9 percent safe. That doesn't mean a team with a 10-point lead and the ball with 10 minutes to go has only a 9 percent chance of winning. Rather, it means they're 9 percent of the way to having a completely insurmountable advantage.
An 11-point lead with nine minutes to play—we'll let you keep the ball. That's an 8.5-point safety margin with 540 seconds to play; it's 13 percent safe (72.25 divided by 540).
A 12-point lead with eight minutes to play ... that's a 9.5 point margin. It's 19 percent safe (90.25 divided by 480).
A 13-point lead with seven minutes to play ... 26 percent safe.
A 16-point lead with four minutes to play ... 76 percent safe, assuming the team with the lead also has the ball. It's really unusual for a team to come from 16 back with four to play and win, but it does happen. I would guess it happens twice a year somewhere in the world of college basketball.
A 17-point lead with three minutes to play ... bingo. That's a safe lead. Seventeen points with three minutes to play is a safe lead whether you have the ball or not, actually; a 17-point lead with the ball is safe at 3:30; a 17-point lead without the ball is safe at 3:02.
Once a lead is safe, it's permanently safe, even if the score tightens up. You're down 17 with three to play; you can make a little run, maybe cut it to 8 with 1:41 to play. The lead, if it was once safe, remains safe. The theory of a safe lead is that to overcome it requires a series of events so improbable as to be essentially impossible. If the "dead" team pulls back over the safety line, that just means that they got some part of the impossible sequence—not that they have a meaningful chance to run the whole thing.
Why calculate when the lead is safe? The real answer is "because I like to." I like to feel that I understand little things about sports. I like to feel that I can see the difference between a safe lead and a live contest for the same reason that I like to feel that I can recognize a zone defense and recognize a pick-and-roll.