The oxygen-hacker -book
Vance Harlow's OXYGEN HACKER'S COMPANION From AIrspeed Press
has some nice ideas about continous mixing trimix.
You do not need an He-analyzer, but with two O2analyzers, and some math, you can measure the mix correctly through a mixing-stick, that you can build yourself.
This will enable you to top off your mix-bottles.
You can also topp off with
1. bagging pure He in your bag, through the compressor, then
2. top off with 40% -that you continous-blend with a simpler mixing-stick, and
3. a little air on the top.
This is what I do... -when not using the booster
jon o