I only raised the issue of a far-forward heel to show that my 'formula' was not foolproof - one cannot increase the distance (between heel tip and wearer's heel) without limit. Sorry if this misled you. And I really only homed-in on the back of the wearer's heel as a datum because this can be established quite easily, whilst the back of the shoe or the 'ball' of the foot could not be.
I'm not suggesting the Zanotti would be difficult to walk in (compared with a more conventionally placed high heel). I am pointing out the obvious - that if the (raised) heel is so far forward that it causes the foot to tip backwards, then walking is more or less impossible. This would (I hope) never arise in practice as, aesthetics aside, such a design would be pointless - although nearly flat 'negative heel' footwear does apparently exist. If one thinks of the foot as a potential see-saw, the fulcrum (heel) must be far enough back for the wearer not to fall backwards; his (or indeed her!) centre of gravity must remain in front of the fulcrum. I think the 'heel-less' shoes (which look hazardous but are, I gather, not hard to walk in) demonstrate this - unless one leans backwards and the centre of gravity becomes shifted behind the heel tip.
I can now see your point about the relevance of the 'ball' - or, rather, the point at which the shoe sole parts company with the ground, as you now put it. That makes sense and I think you are right to identify (as we now do) that the key obstacles to walking in a high-heeled shoe are related closely to the point of impact of (a) the heel tip on the ground; followed by (b) the sole on the ground; with (a) and (b) then repeated. And these repetitive actions are themselves influenced by (i) the height of the heel (as it changes the geometry of the foot and thus the potential points of impact); (ii) the positioning of the heel (as it also changes the point of heel impact, regardless of the foot length) and (iii) to a lesser extent, the length of the toe of the shoe (as it effectively lengthens the foot and thus the potential 'take off point' for the next step).