It's beyond me, but with MudGuard's formula, aren't you subtracting 6.5% of 100, which isn't what 6.5% of a lesser amount (the sale price you added it to) would be?

I mean, you don't need to know what 6.5% of 100 is.

100 = X + (X * .065)
X = 100 - (X * .065)
...then my brain goes dead.