Do a Flight like you normally would..
after you land measure the fuel you have left
Look at the Telemtry fuel remaining

let's say your pump factor was 350 and your fuel left was 58% but your Telemtry said 48% left
use this formula

350 x (100-58) / (100-48) = 282

this example your New pump factor is 282.

each pump will have a different pump factor. Even 2 same class engines.

hope this helps