For many years, with 4-engine piston, turbopropellor, and transport jet operations, the PNR was calculated thusly...
Time to PNR = E x H / O + H
O = ground speed out
H = ground speed home
E = endurance
Endurance was the total fuel on board (in hours) at the normal cruising fuel consumption,
minus 1.5 hours reserve (or, 2 hours, if using island reserve).
Worked very well in service.
I never
ever departed with less fuel than indicated above, and never had any arguments from management, either.