It would work as a brake but your suggested BOSS settings wouldn't be valid anymore. Trying to set up BOSS without some kind of guidance can be a real challenge. I'd just keep on looking for the the regular BOSS brake. Sooner or later I think you'd find one.
I can understand wanting a brake on a .300, but the danged things are too freekin loud for me. I'd rather put up with the recoil, unless it's one of those rifles that beats the snot out of you because of the way the stock doesn't fit you. Been there and got the teeshirt.
An alternative, to get you by: the PAST recoil shield. They work well and don't make your rifle any louder.