Max Lobe Ramp For Solid Flat

Performance trends has a program called "Engine Analyzer PRO" which has a theoretical minimum tappet diameter calculator that seems to be consistent (+ or - .001") with lobe grinds from several different cam manufacturers catalogues.

It takes into consideration the following.
Duration at any two of three lift specs (seat to seat, .050" or .200")
Max lift
Lash

I wish I knew the formula it uses to come up with these calculations.

Unfortunately the program itself is marketed at commercial businesses and is VERY expensive to buy legitimately.