I think you're on to something! Forget metric vs imperial... how about a BINARY SPG?
Needle stays at "1" as long as there's any air whatsoever in the tank, and doesn't move until you are completely out of air... at which point the needle suddenly swings to 0. What could be simpler?
I have seen many divers using exactly that. In their case, green is good and red is bad. Or, maybe it is really a ternary SPG, green=good, yellow=time for safety stop, red=bad and I may die.
On a number of trips on the boat after a dive I have heard people talk about how they were still in the green and could have stayed down longer. Or, they had to surface quickly because they were in the red.
I don't care if you use metric or imperial, just use something.