Hi,
Why don’t you determine it empirically — as presumably it is the position where aberrations are minimised?
Star test either side of a know positions further and further out from location of max in-travel. Use a digital video camera to capture the out-of-focus start images and allow you to average them and you’ll have a record of the intra and extra focal images for each mid-location. Best position is where the intra and extra focal images are most similar to each other.
Cheers
Martin