I used some of that useless high school geometry i learned. I took the original 97" length + 65% of the circumference of the new pulley ...
Anyway I figured it would add about 20" to the length. I wasn't too far off with my math. But I really did do it by the numbers.

Then afterwards when getting a new belt, I stapled two belts together and wrapped it through the pulleys and then subtracted 1" for tensioning.