5% might get you what you need!
I don’t know why but a 7.5 kw motor is = to 10 HP. The transformer has to be sized by the table in the NEC for 10 hp 3 phase at 220v
27 amps at 220 v 3 phase = 27A * 381V =10.287 KW.
The boost is sized to the percent of voltage boosted. So a 10.25 kw load boosted 10 percent voltage would only need a 1.025 kw. You might be able to get away with a 1 kw but it is right on the edge and I like to round up.
I’ve installed these on a bunch of equipment from overseas and never had any issue. Sometime I just wire them into the cord set on mobile equipment.