I do that in my head constantly throughout the dive. Although I admit I cannot dive in Imperial.
I know my tidal rate is 18l per minute. So 36l at 20m.
I then double it to allow for increased workload.
My normal buddy (SWIMBO- has a much lower rate than me and normally dives a smaller tank.
Our predive planning aleays includes my gas failing amd us bailing out onto her tank. Again allowing for deco and increased consumption.
Always plan plan plan...