Refer to bg wiki inundation for further info on how 12.8% increase is attained, it doesn't show where numbers are floored however it'll explain.

"example(Current WS dmg (5000dmg) x SC level multiplier 2.25 (Which is a 6th step double darkness or light (current SC dmg 11250) x Inundation level multiplier 6 (67500) = Final SC damage {67500)"
you don't multiply the inundation with the SC multiplier, you multiply the dmg result of the SC multipler with the inundation multiplier which results in the final number being approximatly 12.8x the WS damage. the math in there has something wrong a little, but it explains the 12.8x