I'm not saying my way is 100% perfect, just works for me. I have 15+ gas mags between all my pistols and GBBR's and I have had less than a 2% failure rate since I owned them. I keep my mags filled with lubricated propane to approx 30%-40% full. I don't have the amazing gas efficiency of your mags so when I expel all my bb's I know my mag is pretty much empty on propane. ill give the mag a quick 2 second squirt before storing them for the next game.
I DO purge my magazines, I try to not do it all the time because its not good for the seals. if you fully fill your mags with propane and you are getting 4 mags of bb's maybe you should be filling them up at 50% so you have less gas to waste after a gameday. Or just store them with the current amount of gas left in them (wont hurt them)
if you are using propane, make sure you are lubricating your mags by having a oil reservoir on your filling spout. I give 1-2 drops every 4-6 magazines. I find that if I over lube my mags that the lubricant gets on the hop up rubber and your accuracy goes out the window.