question

Battery Voltage is not used in the BMV's SOC calculations at all. Using Voltage to estimate SOC is a very coarse method and is dependant upon so many different factors that it's almost useless.
The BMV calculates SOC by counting the energy going out from and into the battery.

@karnes314

Victron won't be using Ah directly to calculate SOC. Ah needs a Voltage at the time of discharge to be meaningful as energy.

The Charge Efficiency is fairly straightforward, for you every 99 Wh counted going to the batts needs 100Wh applied.

When discharging, the Peukert Exponent is applied. This is (sort of) a measure of efficiency, and if you discharge quickly you'll pay an SOC penalty. If more slowly than the batt C20 rate, you'll see more SOC remaining than you might expect. Might be hard to get your head around this, but you could google re Peukert, and there's a bit in the Shunt manual. After some discharge you can even change Peukert to see the difference it makes - this is reversible, so you can revert and it will recalculate.

In practice you can adjust either/both to control day-to-day 'drift' in the SOC at a fully charged state, but you'll need to gain some mastery of doing this. And you may even find the difference in batt Temperature between seasons requires a tweak.

This drift is of course controlled/repaired ultimately by the sync to 100% function, and you may have to set up the Tail % and Charged V settings to have that happen when you want it to.

Have fun..