It's very simple. Measure from the center of the axle to the center of the brake mounting bolt, then subtract either 315mm, or 311mm for 27" or 29" (700c) and that's the brake reach needed. Actually, you generally need 1-3mm more than this calculation because the brakes aren't aligned radially, but it's close enough to estimate which brakes are needed.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.