Jamie,
I seems as there is no spec for the probe location on the pipe as its location would change in relation with the piston with the length of flex used. Only the size of the hole is given. Ask your engine builder or take consensus of the answers to follow.
I ran mine closer, right at 3 1/2 inches from the piston face. It really doesn’t matter much, as far as getting accurate readings, but if you want to compare your EGT readings to your friends, your probe should be very close to what their running them. The farther from the source, the lower the readings. I also think the end of the probe should be in the tube center. Bottom line, just to it like everybody else.