Our company allows 3% of the higher number. Best way to work it out is to multiply the higher figure by .97, if the result is less than the other figure, you're good to go.
Example:
Fuel from truck plus fuel remaining from previous flight = 10,000 lbs
Fuel on gauges = 9,800 lbs
10,000 x 0.97 = 9,700 lbs
9,800 is more than 9,700 therefore 9,800 is within 3% of 10,000 and the fuel check is ok.
Multiplying by 0.97 is the same as taking 3% off, it gives you the lower limit of the 3% check. If you need to do it the other way, multiply by 1.03 that finds the upper limit of the 3% check, however if your rule is like ours, within 3% of the higher figure then you would always multiply the higher figure by 0.97 not the lower figure by 1.03.
If your rule is like BOAC's, multiply the calculated uplift by either 0.97 or 1.03 as appropriate depending on whether the gauge figure is higher or lower than the calculated figure.