This post is showing how to find an average if you have sum of 2 columns in 2 different tables.
For example, I have Sales 1 in Sheet 1.
I have Sales 3 in Sheet 2.
You will think to create a Measure to SUM the 2 columns as some of the posts show you. DON’T, it does not give you error but the figure will never be correct.
You need to create New column not measure but using the same SUM function.
CSales = SuM(Sheet1[Sale1]) + SUM(Sheet2[Sale3])
Then I create a measure like the one below.
Measure = (SUM(Sheet1[Sale1]) + SUM(Sheet2[Sale3])) / SUM(Sheet1[CSales])
This may not be a perfect way but it works for me after trying a lot of methods the Internet suggested.