The rules I cited in post 14 are not the IPSC current rules. Indeed, I am not immediately relocating where I found them, but I'll leave the post up for anyone shooting a match with the other PF qualifying rule. The current IPSC rules say 8 rounds of a competitor's ammunition are collected at the beginning of a match. One bullet is pulled to determine projectile weight. Three are fired to determine if the average velocity makes the power factor the competitor is claiming. If they fail, three more are fired, giving the competitor a second chance to pass.
Statistically, that produces a different outcome from the rule I looked at. Suppose your average velocity was right on the power factor number. As you can imagine, a single set of three then has a 50% chance of passing or failing. But when you get a second chance, your odds of passing increase to 75%. This is because two chances have four possible combinations of passing and failing. Pass and Fail, Fail and Pass, Pass and Pass, and Fail and Fail. In only one of those four outcomes are both tests failing, which would have to happen for you to fail to meet PF. The other three give you one or two passing results, so with only one-out-of-four failing, your odds of failing are only 25%, so your odds of passing are 75%. What this means is that to have a 50% chance of passing from two tries, your average velocity will actually have to be below the power factor number.
The second difference is what determines whether or not the average is likely to equal or exceed the PF is not the standard deviation I used in post 14 for the rule given there, but rather it is the standard error. Where standard deviation tells you how much to expect individual shots to move around an average value, standard error tells you how much to expect the average itself to move around among different test sets of shots. It is equal to the standard deviation divided by the square root of the sample size. For 3-shot velocity samples, it will be 0.577 times the standard deviation.
Based on the above, here is a table for the two-tries at 3-shot passing averages, assuming you know both your average velocity and the SD for your load. Again, you do best to determine your load's average velocity and SD from a single sample of a larger number of rounds. 30 is recommended. Again, I'd add 10 fps to the result below to allow for chronograph and distance variation.
Code:
Chance of Average
Making PF Velocity
50.00% -0.315 SD's
60.00% -0.195 SD's
70.00% -0.069 SD's
75.00% ±0.000 SD's (average right on PF velocity for your bullet weight)
80.00% +0.077 SD's
90.00% +0.276 SD's
95.00% +0.439 SD's
99.00% +0.740 SD's
99.90% +1.068 SD's
99.99% +1.317 SD's