I don't think you can work out cfm from throat diameter, you would need to get the carb on a flow bench.
Just to confuse everyone us all, a few facts on carbs sizes etc.
The old calculation of cfm= d x rpm x ve divided by 3456x100 is actually rubbish. (this is usually quoted in most holley books as a way to judge required carb size). The problem is that 2 and 4 barrel holleys are measured cat either 1.5 or 3 inches of Hg (vacuum). A performance engine when being turned hard has bugger all vacuum high up in the rev range, try around 0.5 HG. Thats why race V8's regularly use 850 / 1050 rated holley's when the calculation will actually come out at around 640 cfm (assume a 350 ci chev at 7000 rpm @ 70% VE).
To confuse us all holley doesnt even rate their 2 and 4 barrel carbs the same way, 2 barrels are measured on 3 inches of vac, 4 barrels at 1.5, so you can see a 500 cfm 2 barrel holley and a 500 cfm 4 barrel, WILL NOT flow the same.
Manifold design also has a huge impact on carb sizing, a seperate runner design like we all run on pintos will need say 2 x 48 dcoe webers to put out say 200 bhp plus, but the same 2 carbs on a single plenum manifold on a 350 chev will put out well over 500 bhp. Conversely a single 32/36 weber (way less flow that a pair of sidedrafts a shared plemum manifold (stock carb and intake for a pinto) will produce good power, up to around 120 bhp plus pretty easily if the head, cam etc is all done right.
Confused????? I am .....
Shaun B