Accounting for the effects of THRUST REVERSERS on DRY runways is not permitted, however, the FAA have only recently started to account for the effects of WET runways.
The B747 was certified under the older regulations where the WET/CONTAMINATED takeoff information supplied from Boeing was "ADVISORY", this information used FAA AC-91-6B as the basis for using a 15 feet screen height and THRUST REVERSERS for stopping.
The B747 information specifically states that "STOPPING DISTANCES ARE BASED ON THE USE OF SYMMETRIC REVERSE THRUST".
We went further with the MEL and turned their “not recommended” into a “prohibited”.
777AV8R, the WET/CONTAMINATED data for your machine was based on the use of One Reverser. The penalties for zero reverser thrust are quite severe.
Mutt.