If you look at a typical Li-Ion discharge curve then 4.2v down to 3.7V is pretty much the useful life of the battery (90%). Looks OK to me so why complicate things?You are proposing basically using a voltmeter to estimate the capacity of LiOn battery. Since the discharge curve of a LiOn is basically flat, this is a very poor indicator of state of charge...
Gadgets that use LiOn have a microcontroller on board that actually integrates ∫i(dt) to report remaining charge. Save your time, or do it right.
So if you want to do it right, what would you do?Save your time, or do it right.