gardner:
The type of battery chemistry is not mentioned. I once did a state-of-charge model for flooded lead-acid chemistry and you can look at the details here:
Estimating state of charge for lead acid batteries « ProjectsThe curve mrjonny2 posted does not look like lead-acid to me, but you could use the same method to bodge up a calculation.
From the voltage range on the chart it appears to be a Li-po cell, to me anyway.
I did mine more for interest than anything. I don't think state-of-charge modelling is particularly accurate.