Use a FIXED LOD for the denominator (which is SUM([Sales]), right?)
On Sheet 1 I did that using month and year in the LOD as the level of detail dimensions. you can see the numbers match. But as you know, order of operations puts the filter evaluation after the FIXED LOD.
However that changes if you put a filter in context. In sheet (2) I move the three filters that need to impact the denomination in context. (Right click the filter and you can see where that gets set.) When a filter is in context, it gets evaluated before the FIXED LODs. The 4th filter remains out of context, so it does not impact the LOD.
Thank you for your help. I didn't think it was possible. I appreciate your help.