New way to measure belts after pully swap?

so guys, im sitting here, bored, which is a bad idea.. i got brainstorming (even worse idea) and thought about belts.....

I am considering pullyswapping my bolens 1055, and was trying to work out what size belt to get. i got thinking of a 'new' way to measure.

Know what size belt you previously had, now measure (or know) what size pullies you are swapping on. now, subtract half of that pully diameter from your old belt, and theoroetically, you'll have the new belt size.

Example-
Starting with say.. 35 inches, just cause thats what i have.
Now i swap on a 5 or 6 inch pully.
now, using my "formula", i subtract 2.5 (5 inch) or 3 (6 inch) from my previous belt
By this math, i will need a 32.5 (5 inch) or a 32 (6 inch) belt..

What is your take on this idea? do you think it would work? i have yet to try it, so i wouldnt know. it SEEMS like it would work though... let me know what you guys think.. maybe im just cray cray   

are you putting on a smaller pulley? if you are, that would work. if you are going bigger, you need a longer one.