What is the formula used to determine blower fan size given total air volume and system BTU capacity?
Unless your house is very small or extremely energy efficient I think you should read the book again and recalculate the size of the heat loss and heat gain. The 400 CFM/ton of A/C is accurate enough for me. Sizing a furnace is not rocket science and is not an exact science since there are too many variables. Years ago when I was still doing this work (before retirement) furnaces were sized on their input or output and the size of the drive required. Do not try to be too exact unless you know "exactly" how the house was built and can guarantee that it was built to those specifications. As far as the "proper formula" used I have no idea, it never had to be perfect.