The manual sets out a chart for boil off, I believe. In my experience, I get the wort to a boil and then "simmer" it at 50-60% power. I get a bit less than 1 gallon of boil off, typically.
OOPS - I confused this with the mash sparge chart. It is in the text of the controller operation section - 240V boil off at 100% = 1 gallon per hour; lower percentage of power results in lower boil off rate.