Charts can be helpful, but keep this in mind:
- The chart generally applies to one bat. temperature.
- Many charts assume the battery is being neither charged, nor discharged (like for an hour or more, nothing in, nothing out). Or, they will give you two sets of graphs, one for charge curves at different rates of charge, ditto for discharge curves.
- The chart assumes your battery pack is brand new. An old tired pack will not follow those charge/discharge curves like a new one.
You can also check electrolyte density, which tends to be a bit more reliable, especially if you use a good hygrometer.
Overall, the best method is probably an accumulating amphour meter like the trimetric. It it not perfect, but it's very very good.
Good luck, have fun, play safe!
troy