Checking an SPG to monitor expected gas consumption hardly rises to the level of a math calculation. It's more on the level of a visual observation.
I've always regarded the printing on an SPG as largely irrelevant, since it's really the movement of the indicator needle that's significant. That's why there are 'dual gauges', which are nothing more than the same old gauge with two lines of print. No matter how you slice the loaf it's pressure that's beintg measured. The fact that you can paste on a different set of numbers and convert an imperial gauge to a metric and vice versa illustrates how fatuous this issue really is.
I've advocated the substitution of smile/frown faces for metric or imperial numbers. At the highest pressure levels there is a bright smiley face (HP tanks will cause the gauge to absolutely beam when full), which gradually changes to a neutral expression and then to a frown of increasing severity as pressures drop.
This will work with dual tanks, small tanks, and large tanks perfectly well. The planning implications of the volume of gas you start with is your responsibility, since it's a fixed and known quantity.
Seriously, scuba related physics are really quite straightforward and readily understandable. For reasons I can only guess at some people like to complicate things to the level of a GRE math section problem. Perhaps it's another case of technical jargon masquerading as complex content.