However... have no fear! Because since the days of BSI there have been two others indexes that are -very- effective and are in fact used by the game to determine the outcome of battles.
Effective Attack (eAttack) and Effective Defense (eDefense).
Those purists that you bring up would tell you that -those two- have -always- been the way to determine battle strength and that BSI was just a silly unimportant number.
Of course you are familiar with the formulas?
eAttack = attack + attack rune + attack IA + gear attack + general attack + 0.7 * (defense + defense rune + defense IA + gear defense + general defense)
eDefense = defense + defense rune + defense IA + gear defense + general defense + 0.7 * (attack + attack rune + attack IA + gear attack + general attack )
Those numbers are then multiplied by any piercing and resistance stack totals to determine the the bonuses for each of those stacks which are then added to the eAttack and eDefense to create Total eAttack and Total eDefense.
You can't just determine the bonus for each stack in your keep and then say that your Total eAttack/eDefense is X.
Since the number of opposing Resistance/Piercing influences the your final stack numbers you would be talking about Total Potential eAttack and Total Potential eDefense when you add your total stacks to your eAttack/eDefense. Of course all said and done your eAttack and eDefense totals are actually ranges that range from eAttack/eDefense to Total Potential eAttack/eDefense.
Finally, I have seen some do eAttack/eDefense calculations and then divide by 100 to get what they called a "beast rating". Such ratings, while diluted from the actual eAttack/eDefense stats, are still a -lot- more accurate than BSI calculations could be since they are (obviously) all inclusive.
Bottom line is that you didn't -need- to try and mutilate the BSI calculation into something it was never intended for. You could have simply used the formulas that existed next to it to get the number you were -actually- after.