Mike,
There is a tie-in from the MEA position to the MEA comp file. There is not a tie-in to "machine zero" (which really doesn't exist on Sheffield machines). The initial offsets to Tip 0 is indeed to the center of the Z ram & flat of ram in the Z direction, however these do not relate back to the comp table, only the position of the cal sphere at MEA position relates back to the comp table. So what you get if you cal Tip 0 at any other position is a mis-placed comp table. Once you do Tip 0 at MEA as long as you tell the system that you are moving the sphere & that it's not @ MEA, you get to do Tip 0 again to tie the new position to the distance from the MEA position. It's really not too bad to do, lots better if you write a program to cal all the tips you're going to use. Hope this clears it up to some degree.