I seem to recall that during (jet) flight tests under the old BCARs the testing required:
Vat + 15 kts on a wet runway, or
Vat + 15 kts on a dry runway plus an added 15% wet runway factor.
Both conditions required normal retardation, i.e. no reverse!
I believe this created the 1.67 and 1.92 factors over Normal Vat at THR.
The boffins would normally choose the smaller end-result as the Flight Manual starting point. Remember in the UK we always were deemed to land on a wet runway........
Under FARs/JARs the Dry runway option appeared, all aimed at increasing the Max Perf Limited Ldg Wt off critical runway. Some countries did choose FARs as the certification standard for precisely that reason.
IT
