The state of charge is very close to linear, but SoC (state of charge) and battery health/capacity are two different things. Also SoC tables for flooded cell (e.g. car batteries) and starved glass mat cells (e.g. the FIRST MK batteries) are slightly different.
For battery health/capacity we bought a pretty cheap static 50A battery tester from
www.harborfreight.com ($10, item 93784). We added a set of anderson connectors so we can simply plug in the battery and read the voltage which provides an indication of SoC. A flick of a switch provides a short 15 second test to determine health/capacity.
For example, we had a battery from two years ago that showed good SoC after charging overnight. However it failed the load test. Its voltage dropped to 2-3v with a 50A load. This could result from a high internal resistance which may occur due to large sulfate crystals forming. Large crystal size is associated with storing the battery with little or no charge for a period of time. Who knows, all we do know is it passed the SoC test but really failed the load test.
Often after charging there is a surface charge which results in an elevated voltage and an incorrect SoC evaluation - a quick 15 second load test removes most of that and gives us a rough indication of battery health at the same time.
Team 1073 Battery Pamphlet (chiefdelphi white paper section)