Need help with a calculated item in a pivot

I have a pivot with data that looks like this. CY and LY are from my data set and CY-LY is a calculated Item.

CY LY CY-LY
Country 1 Measure 1 20 10 10
Country 1 Measure 2 30 20 10
Country 1 Measure 1/ Measure 2 67% 50% 17%


I want another column which is the % difference between CY and LY. It's easy enough to do, just use a Calculated Item and do CY/LY but I don't want it to calculate for the Measure 1/measure 2 as it would be the % change for a % difference which isn't useful to me.

Is there anyway that I can get the calculated item to do a % difference but only for Measure 1 and 2? I would guess an if fucntion but don't know how to write it so that it looks for a %.

Thanks