When setting up a buck and boost transformer, should the voltage setting be lower, or higher than the desired amount? I have to make sure a pump has exactly 220vac supplied to it and last time I checked the service at this place the voltage 120/240 (but the actual voltage hot-to-hot was 249vac. Should I go with a 10% setting or 15% setting on the buck and boost? And which one's work the best?