It's not clear to me if you temp correct for voltage. No mention. I think SG would have to be. I imagine the formula works in some band of conditions. No mention of how wide. It's interesting in the implications for the rest voltage for some batteries. If you knew the batteries very well and the voltage came up on an accurate voltmeter to exactly what you expect, odds are good SGs in every cell are close together and close to nominal.
Once the voltage is a bit low, now the mystery gets geometrically deeper faster without a hydrometer.
Jim
