This isn't quite what you're asking for, but for a rolling 10 month average, for example, you could get an average of the months so far for the first 9 months. Rather than doing the accumulation, do this:
That says you want the average of the current row and the nine rows above it. For the first nine months, it will only average the rows it can find.
Unfortunately, I haven't thought of how to actually exclude the first nine rows since they're used in the accumulation.