Idle N1 on the 747-8/GEnx is ~21% - several percent lower than the idle N1 for the 747-400 PW4000 or CF6 installations (~25% N1). 40%-50% N1 sounds extremely high for taxi.
Some 747 pilots will push the outboard engine up a bit to help rotate the aircraft in a turn, but even then over 40% N1 sounds excessive.