In my opinion all the readings you get from any BDI are just guides. The only accurate way to ascertain any batteries state of charge/discharge is with the SG. With any BDI the accuracy will change during the batteries life and will be greatly affected by the way the battery is being used. Too much faith is put into BDI readings and not enough time spent taking the actual SG's of the batteries involved. The BDI is just an indication of voltage and this can vary greatly from the true state of charge/discharge.