Hello Johan,
I also have a 12L scuba tank. From full I get around 45 - 50 fills on my BSA Ultra. My air resovoir on the Beesa is quite small for a PCP rifle but I expect it's not too different from yours so expect plenty of shooting from a 12L tank. I get a minium of 40 good shots from the Beesa so that's around 1800 odd pellets :)
You are very right about the weight of the tank. When I get mine re-filled my old BMW motorcycle rides home on the back wheel alone.....hehe. One day the cops will stop me!
I am interested in your new rifle so please post a report when you have it.
Good shooting and best wishes,
Alan