I'm not an expert here, so I might be wrong, but afaik the voltage change is not linear. It stays pretty constant for a long time (70% or 80% of the discharge process?) and then drops relatively fast during the rest of the discharge process. At least this is what I see in my netbook.
Maybe the Mugen battery behaves differently, or maybe this non-linear function can be modeled easily, but it's not as trivial as looking at the charge.