To save o-rings and help that spewing problem, turn it until you think the gas is about to come out. Then shoot until it sputters. Then unscrew it the rest of the way. If it doesn't sputter after 5-6 shots, unscrew it some more, then fire until it sputters.
spinnin faster doesn't matter the co2 comin out is the co2 that was in your gun already you can't do anything about that unless you make a machine that pushes co2 from your gun back into the bottle.... but HP will tell you I am wrong because I might be
The amount lost depends entirely on how much space you have in the air system between the tank and the valve. If you use the bottle as a vertical handle very little will be lost compared to using an expansion chamber/LPC, SS lines ect. Using a remote would cause the most gas loss. Most of the time it isn't much to worry about. (only 3-10 shots lost unless using a remote) People running 12grams on markers would want the shortest distance before the valve since 5 extra lost shots means a lot when you only get 30!