So I usually mix 32% EAN in 120cf hp tanks by CB, I f I want to mix 32 for the next dive no problem, but if I want a different mix then I have to either drain the tank and fill or guess what to top it off with. I know there must be a formula for what % to add to what % but I cant figure it out. Anyone have the equation to do that? Say that I have a 120hp that has 1100 psi of 32% EAN but next dive I want 28%, what % would you add to the 32% to come out with 28% at 3500 psi.