Read time readings would be... Well, battery drain overkill, not to mention waking CPU

As for applets, don't get me wrong - Your idea is very good. It's just that Advanced Power Monitor disables vanilla monitor counterparts (thus they don't eat cycles/power), replacing it with own ones, + don't use widget space/resources, + contain many, easily switchable reading modes, including Your algorithm (also switchable between calculating on bme or bq27200 voltage readings).

Of course, one of best joys of FOSS is Freedom of Choice, so I'm not "pushing" you to switch