Thanks @Frank-Scholer, that does make sense. I’d say that’s a pretty significant error in the manual, this is the measure of air remaining after all!
Agree with you re. the current arches not being helpful in general. Since I have to choose between compass heading and tank pressure display, I’m just trying to make use of the arch as a workaround to that limitation. The more I think about it, placing this cognitive load on the user during a dive seems quite problematic and a questionable design decision from a safety perspective.
A graphical compass (with the heading numerical value also always displayed) would be a much better use of this space, with the tank pressure value only shown numerically (at the same time as heading!).