The thickness of the ivory was the accepted depth of movement before the contact `made'. I remember that Peter Hurford and Francis Jackson had totally different ideas of where the contact point was to suit them which could mean quite a lot of work resetting things to accommodate them - and as was pointed out, with mechanical action you simply accepted what was there and got on with it, as indeed do most professional pianists.
Key Contacts....
in General discussion
Posted
I too was told to set contact "make" at the depth of the ivory - not very scientific when ivory thickness can vary from organ to organ.
At All Saints, Leamington, Peter Hurford thought the contacts "made" too soon.
He preferred the make point to be lower so that you didn't have to lift the fingers so high in trills, etc.