I wanted a number that would show how the account is doing overall when I have a lot of very different CPA goals. I have items that are good at a $15 CPA and some that are better at $5. Also my average cart size is important. A $5 item can actually afford a $10 CPA if the cart size ended up being $80.
I just divide CPA by Conv/Value and get a percentage. This shows what percentage of the conv/value I spent to get it.
If I spend $10 to make $80 it is 12.5%.