The only way to calculate it accurately is by basing gas use on your travel speed or "arc on time" per component.
If your gas flow is 6 litres per minute and it will take you 10 minutes to weld a component you will use 60 litres or 0.06 cubic metres of gas.
Many people claim to have come up with an accurate means of linking gas consumption to filler wire used, believe me, they are just estimates.

Cheers - Bob