The lot size was listed as 90,915 sq, ft,. but that was before the transaction that added 232 W 58th Street (Beethoven Pianos) to the lot. That probably added 1,200 sq. ft. to the lot (assuming 20 X 60).
The building looks more like 16 feet wide to me, but the demo permit says 20 feet of street frontage.
Hard to say how far the Beethoven Pianos building does back from the street:
So the question is: how much additional floor space can Barnett add to the tower from the Beethoven purchase? He paid $25 million, so he probably has to add another full floor to the building to re-coup that. If the FAR is about 13.5 right now (90,915 X 13.5 = 1.227 million, very close to the stated 1.211 million sq. ft. from Extell's pre-Beethoven filing), adding 1,200 sq. ft. to the lot would add 16,200 sq. ft. of floor area. That's equal to 2 floors of a tower 90 feet square (similar to 432 Park in floor plate). Two such floors added to the top of the tower would certainly re-coup the Beethoven investment many times over.
Of course, Extell might decide to allocate the additional floor area differently.