That's correct. And the trannies I've tried all run quietly and coolly given a reasonable specAh, I remember now. So assuming your nominal input voltage is 240, the bucking trafo that you buy is a 240V primary, 30V RMS secondary, and you take the 30V off your measured input voltage which in your case is 249. If you buy a 20V secondary then it will take 20V off, so converting 240V to 220, is that right?
I have measured my voltage, I frequently get 250-253 when the maximum legal is 254, so taking 30 off as you do makes perfet sense.