Standard Deviation from varying starting points

Hello again,

When I've posted in the past, I would normally get a solution relatively quickly.  Since this has not happened yet, I'm guessing with the way I presented the question below, a solution is not possible.  That said, I am rephrasing the question in a way that I believe there is a solution based off other questions I've seen solved.

So in the previous screenshot, I had converted the dates into number of months from the current month and pivoted the data.  Here I have an unpivoted table where in another query, I found the min month of "non zeroes" by region and then merged queries with this one.  The "calendar year / month" column has 36 Months of data points while the "St Dev Lookup. Min Month" references the first "non-zero" month that occurred.  Can anyone tell me how to do a standard deviation on the "Test" Column where it only starts counting at the "St Dev Lookup.Min Month" occurrence?  And by the way, I do want to include any zeroes that occurred after the Min Month.  Kudos to anyone that can help and thanks again in advance!