Dean,
I have the benefit of the batteries being new, having the equipment in the box for battery monitoring and still being a land lubber with too much electrical test equipment on a work bench.  First I determined initial battery voltage of course...simple enough.  However, this does not tell you much because Li batteries do not act like the lead acid batteries we are used to dealing with and relating V to SOC is not linear.  But, I have the Smartshunt, charger and CERBO.  I took my cute little charger and charged the batteries to max capacity based on the Victron datasheet, driving through the SmartShunt, a datalogger and Scopemeter.  I then discharged the battery to the original shipped-to voltage using a bench inverter/charger + space heater + Smartshunt + datalogger + Scopemeter.  With the cycle and the data supplied from the Victron datasheet, the CERBO calculated the SOC with the Victron algorithm.  I could also manually calculate total battery capacity from the discharge/charge cycle data and extrapolate the data to verify battery capacity matched the 200Ah promised for the premium paid to Victron (my real motivation).  I could not bring myself to discharge the batteries beyond the storage voltage given the capital I have tied up in the batteries.  So I settled on the calcs for satisfying my curiosity.  I then repeated the charge/discharge cycle with the other 5 batteries. 

I want/need to pull the batteries back down to the storage SOC, as I will not be installing the batteries for a few months yet.  

Matt