How can I be certain of the pile lengths determined by PIT?


If the wave speed c is assumed, then the accuracy of the determined length (assuming also that you see a clear toe reflection) is the same as the accuracy of the assumed wave speed since you are measuring TIME, and TIME = 2L/c, so L varies directly with c. You should be able to guess the wave speed within 15% and likely within 10% based on the concrete strength. If a pile with known length has a clear reflection from the toe, then the wave speed can be determined. If the concrete is consistent with other piles on that site, using that same wave speed will result in lengths with relatively good confidence.