Can only speak about the airbus here. The distance assumed is the distance to go from PPOS to the TO-wpt + dist to the rwy. Normally you would extend the centreline from the IF or FA(F/P) not the IAF (IAF is typically located outside of the c/l).
DTG/track mileage is not reliable here, so will be the vertical deviation/VTK error. Also consider that the vertical deviation is not only based on a false DTG but also based on the planned (managed) descent speed, you will very likely be flying an ATC selected speed.
What helps for me is to use the 3 times table with reference to the TO-wpt, taking the present altitude minus the G/S intc alt, once turned on base. Allowing some extra miles for speed reduction/wind etc.