With B-B transformers you're never going to be right on the voltage. Personally I would opt for the closest higher voltage that you can zero in on. A NEMA rated motor should operate satisfactory at +-10% of its NP voltage. The motor should run a bit cooler at the higher voltage anyway.
The only uncertainty is how stable the supply voltage is which you probably don't have any control over. If it goes higher than you anticipated when selecting the B-B Xfmr naturally the output will be higher than you anticipated.
So you have to give it your best calculated shot.
