How to determine tripod screw offset of camera grip

[Rai]

Whats the best way to accurately measure the tripod screw offset on my grip (canon BG-E2N) so I can set the amount with the T-adapter? Is the best way to look through the lens (lens end) with aperture set small and line up the bright spot with some sort of ghetto plumb-bob setup? Hoping there's a simpler way to do it....

PS. Bill everything arrived nicely - thanks very much for your help

PPS. before someone makes the comment... even though there is some initial flex with the grip I prefer to live with that rather than have to take it on and off every single time i want to use it on the pano head

[Rai]

Using skills long lost since kindergarten I used some Blue-tack pressed out flat on a table which was pushed near a wall. Using the wall to keep one side square I simply made imprints in the Blue-Tack of the grips tripod screw and hole on opposing sides.

5mm if anyone is wondering.... which is sort of how I roughly measured it by sight anyway.