>OK, I didn't realize that.
>
>After a little bit of algebra, I get the formula, using same variables as above plus:
>
>X = total dollar amount of taxes
>
>I = X / ( P + ( P * F ) + F )
>
>If you plug in X = 74.88, you get I = 500.03339, which is the problem you saw earlier.
>
>If you plug in X = 74.875, you get exactly 500.00, but you have no way of knowing that's the exact value for X.
>
>However, you do know the true value of X must be between 74.875 and 74.885 if it was rounded to 74.88. So, plugging in those two values:
>
>X = 74.875, I = 500.00
>X = 74.885, I = 500.06678
>
>Any original price I in the range of 500.00 to 500.06 inclusive will give you a tax value, when rounded, of $74.88. The value 500.07 is too high, it results in $74.89 when rounded.
>
>So, you cannot determine the exact value, only the range it belongs to. The reason you can't get the exact price value is that the tax amount is only 14.975% of the price, so you have only that percentage of the precision, if both the price and the tax have been rounded to two decimals.

Yes, that seems to be a resume of the explanations I got from Gregory. But, thanks for the additional details

The most important part for us is to extract both taxes from this total amount of taxes. So, that would be exact. I still have more tests to do. But, so far, it seems to be a perfect match. I explained them that it would be impossible to get the exact amount before taxes with just a tax total, if this is in a province dealing with two taxes. To precise all that, they are looking right now at seeing the possibility of having access to another field which would provide the base amount.