Inbalance,
What manual are you getting the performance figures from? Airplane Flight Manual or Airplane Operations Manual?
As per the FAR:
(f) Retardation means other than wheelbrakes may be used if that means -
(1) Is safe and reliable;
(2) Is used so that consistent results can be expected in service;
This basically means that if the aircraft want to certify the aircraft with the use of thrust reverse as a stopping means, they have to prove its reliability. Its quite possible that your aircraft manufacturer certified the use, if so, the figures will be in the AFM, if they are in the AOM, they arent certified figures. You will have a much easier time convincing the authorities if the data is in the AFM.
Mutt