MER

MER is net revenue divided by total advertising spend across every platform, taken from your own store rather than from the platforms.

It cannot double count, because there is only one numerator. That makes it the honest counterweight to platform ROAS figures, which overlap and therefore add up to more revenue than you actually took.

It will not tell you which campaign worked, and it is not meant to. It tells you whether the whole machine is running at a ratio you can afford.

Use net revenue rather than gross on top. Gross revenue still carries the VAT you never keep, which flatters the ratio by the whole VAT rate and makes two months with different return rates uncomparable.