Net reverse thrust in the 747-400 GE installation is, I've been told, about 40%.
The fan produces about 75% of total forward thrust, leaving 25% from the hot section. Theoretical max reverse, then, is 75% - 25% (hot section is still blowing aft), or 50%. Then there are efficiency losses in the cascade setup. Further, N1 speed is limited in reverse to about 95%.
It's a moot point, though, because reverse thrust for backing on the ground is prohibited by our Flight Handbook.