1. The S.G. is checked several times a day. One asks for the S.G. from the bowser driver during every refuel operation. if he doesn't have a current one to hand he can contact the fuel farm direct by radio or phone for the latest reading.
2. The allowable difference (discrepancy) between uplifted and calculated fuel is dependent on a/c type and airline policy. Some airlines use a standard +/-3%, others use an actual figure (Kilo/Litre) based on a go-nogo graph which is usually carried on the flt deck in the refuelling manual.
3. If the fuel discrepancy is out of limits one tweeks the S.G until it isn't.
Ok the last bit's not true, if it's out of limits then a stick/drip check is required to determine the actual fuel on board. It can take a few minutes (737) or half on hour or more if it's a 747.
Hope this helps.
