Standard Deviation in Calculated Field Using Dimension

Hello,

I have a set of monthly data that I'd like to aggregate and calculate z-scores on, which I'd use in a table to show the z-scores for the last month. I can get calculate the z-score on a trendline easily, but I'd like to sort or filter on the last month's z-score only, hence the need for a calculated field. The data is set up in a way that there are multiple rows per month, i.e.:

So far, I've got the easy pieces done:

Last Month: sum(if month="x" then [count] end)

Average: sum([Count])/CountD([Month])

I'm running into trouble with the standard deviation, though. If I use the STDEV function in a calculated field, I get STDEV(1000,1500,1100,1450,1200,1600)=242, which is of course how it's supposed to work. What I'd like to get instead is the monthly aggregation, that is STDEV((1000+1500),(1100+1450),(1200+1600)), or STDEV(2500,2550,2800)=161. I'm sure if I created a calculated field for each month I could get it to work, but I'm wondering if there's a simpler way that I'm perhaps missing.