On a single cone turbine (Merlin, P120 and P160 etc, Booster) the distance between the back edge of the turbine tail cone to the start of the tube (not bellmouth if large spinning) actual tail pipe tube, which on a 160 is usually 80-85mm 3 1/4-3 1/2" dia section. Gap not inside

If the tail pipe has one of the small flared lip then measure to that 25mm gap.

Dave W