Check
B737 Technical Site for fuel gauge accuracy.
According to EASA, final reserve fuel for turbine-powered aircraft is:
1.5. Final reserve fuel, which shall be:
(b) for aeroplanes with turbine engines, fuel to fly for 30 minutes at holding speed at 1 500 ft (450 m) above aerodrome
elevation in standard conditions, calculated with the estimated mass on arrival at the destination alternate
aerodrome or the destination aerodrome, when no destination alternate aerodrome is required.