Weighing after filling is to check how much you have managed to transfer. If you somehow fill too much liquid CO2 which occasionally happens, your pistol will not fire and you will have to vent a bit of gas. Normally you get a vent screw cap with the pistol. I used to fill my FWB Model 2 from a fire extinguisher as shown below. Now fill my Crosman 111.

Baz

https://i.imgur.com/1ADwNgG.jpg

https://i.imgur.com/xTP194I.jpg