Do you have a charger that can stop at 80%? That's where the big gains are, not going all the way to 0% or 100%.
The flat discharge curve for lithium makes it difficult to estimate the charge based on voltage. I used a watt meter like this for my ebike to keep track of how much power I used.
http://amazon.com/GT-Power-Analyzer-...dp/B00C1BZSYO/