Create a calc called [Index] that looks like this:
Now (for the sake of testing) put that on the label so that you can see the values of INDEX. You may need to mess with TABLE(across) TABLE(down) or other settings. What you want is to understand what it takes to get INDEX=1 for the first value, (or to get a value that you can always identify) and then tell the running sum not to evaluate for that value.
If I had a workbook to mess with, I could show you easier than I can to describe it. If you can't understand what I was trying to describe above, post a workbook and I'll show you.
I'm new to Tableau, can you elaborate on how to get the Running_Sum function to exclude Index() = 1?
IF INDEX <> 1 then RUNNING_SUM([Incremental A]) / (RUNNING_SUM(([Incremental B ]) + RUNNING_SUM(([Incremental C]))) END
But again, this will require setting up INDEX so that it runs along the months properly.