The tail clearance on rotation when everything is going well is only a couple of feet. If the rotation is a bit quick, or a bit too soon, it messes up the design geometry and erodes the tail clearance pretty quickly.
Correct. "Textbook" rotation gives 60cm tail clearance for all engine case. 30cm clearance for engine-out case.
Reducing the ATM derate by 3 degrees makes a RR rotation feel a lot more like a GE. So I'm told.