When you get a new belt that fits record the details if you can on the machine in half inch wide permanent marker where the sun won't bleach it out or it can get wiped off. .
I've done the same for my drill press , bandsaw , lathe , engine driven rotovators , etc etc. ,, it's been a godsend several times .
Deffo use the bearing boys already mentioned for your calculator it's perfect especially if you are looking for " matched pairs " or " trio's ) that put a lot of torque on the driven pulley such as on a tractor driven log saw slide with a four foot diameter 1/2 " thick log cutting blade or something on an old combined harvester .
I 've also used it when repairing a couple of lightweight lawn mowers that have ribbed belts to get the precise size .