The problem is Mark, unless your battery is new then how do you know what the capacity is given a batteries capacity will diminish as part of the natural ageing process after the first few charge/discharge cycles
Quality of battery and more particularly how well it's looked after affect the rate at which this capacity diminishes. I've seen good quality batteries ruined in 6 months on a boat due to woefully inadequate charge regimes and at the other end of the scale lasting with useable capacity after 12 years when properly cared for.
Since the voltage measured only gives you an indication of state of charge for the remaining capacity it can only signify the energy remaining if the capacity as input on this device is kept up to date. I hope you can see where this is going since it's difficult to measure capacity without a drainage test under controlled conditions.
ETA: Sorry our posts crossed Bob. Cranking voltage will give some indication of capacity but difficult to quantify given the large number of variables. A drainage test over several hours with a much lighter known load is more meaningful. TBH if you have any doubts about the ability of the battery to start your car and it's more than say five years old then just replace it.
