I dive a Prism these days but I was saying I'm well practised at thinking along the SCR lines ie what the FiO2 drop will be for a given gas at a given flow rate. As an aside I used to dive the Azi as a two mix unit. The deep gas was run at the flow rate of the shallow gas...so something like Nx26 and NX50 running at about 6lpm got me to 50m or thereabouts. Bailout for that was Air and the on board stuff.