Example
Airbus A320 has 4000kgs showing on board before fuelling
Required fuel for departure 13,000kgs
13,000 x 4000 equals 9000 kgs estimated uplift from refuelling vehicle.
Ask refuelling company for the correct SG of the fuel supplied.
For example if SG is 0.789 then the estimated calculated uplift should be 11,407 litres.
Once refuelled, whatever the litres is, convert back to KGs and the difference is the discrepancy.
For example, if the actual refuel is 11,700 litres, that equates to 9231 kgs, a discrepancy of 231 kgs more needed to give you your 9000 kgs extra, meaning your fuel gauges are under reading. I’d say if they are over reading, then that’s more of a problem.
The whole point of working out calculated and comparing to actual is that you are taking the opportunity to use fuel providers calibrated metres to make sure your aircraft gauges are reading correctly.
Your aircraft manufacturer should have tables with maximum discrepancies allowed for certain amounts of uplift or fuel on board before a mandatory fuel tank stick check is required.
The above is only my thoughts an opinions. I was an aircraft refueller for Shell in the UK for 9 years before Commercial Pilot.