That graph is meant to show the useable capacity of a given battery at different discharge rates. The basic gist being, the faster you discharge the battery, the less actual capacity you have. It's not really useful for determining a percentage capacity left over (though there is a correlation).
An 'accurate' percentage is going to be difficult to computer and maintain, and likely not really worth the effort. The simplest method is an approximate percentage based on battery voltage. If you know the average current draw that the battery will be providing, then you can tailor your percentage to that current draw from the above discharge rate chart. If it's a steady state current draw, just use the charts above as is. If it's more of an intermittent current draw, then the capacity will be a bit better than the 'average' current would indicate (as the battery has time to rest in between demands for current, which gives the internal chemistry time to normalize and improves overall capacity)
Here's one thread on voltage to capacity conversions: Can someone tell me LiPo Voltage vs Percent Charge? - HeliFreak
There are others. Again, a simple voltage measurement is only going to give you an approximation of capacity.