Accel and decell are only really a factor on commanded moves because the Mach knows where it is stopping and plan the deceleration to that point. When you probe, Mach has no idea when the probe will trigger. When it triggers, Mach says stop, but it never stops exactly on the mark. That's the same reason the THC commands overshoot. The faster the THC feed rate the more overshoot you have. The solution to both is to turn down the feedrate. I probe at 30IPM with is a acceptable compromise between overshoot and speed. The slower you make the feedrate, the closer you'll come to stopping on the mark.