There is no absolute formula, but Pre-Check seems to move 5-6 times faster than a standard line. That may be due to the shoes, 3-1-1 bag, scanning device and general traveler sophistication.

Thus, I don't care about the length of the line, I care about the time it takes to get through the checkpoint.

My unscientific estimation is that Pre-Check is almost uniformly significantly faster than any non Pre-Check line unless there is a real screwup and those happen in all lines randomly.