Without being able to identify what aspect of BS's calculation is causing this difference, I'm not sure you'll be able to accomplish your objective.
I believe if I use your spreadsheet calculations as a base line I can tinker BS into submission.
Thanks for all your help and I hope your back recovers soon. Ya know down here in Central Texas we use a teaspoon for a snow shovel. We almost had to break out this December but it was a false alarm.