True, the old adage "measure twice, cut once" applies directly to carpentry and not to belt drive design, but there is some truth in the saying that designers should pay attention to.
When designing a replacement or retrofit synchronous belt drive for a piece of equipment currently using a V-belt drive, it is recommended that the actual operating shaft speeds be measured instead of just basing the new synchronous belt drive design on the existing V-belt sheave diameters.
Glad you asked!
V-belts will slip, from little (well maintained) to a fair amount (if poorly maintained, or running in worn sheave grooves). It is very easy to make significant errors in the driveN shaft design speed if you assume that the V-belt drive is operating at exactly the ratio as calculated by the sheave diameter ratio. Good practice is to always measure the shaft speeds - with either a contact or strobe tachometer.
This is of extra importance when designing a drive to operate on fans or centrifugal pumps - where the driveN load characteristic is sensitive to operating speed.