Weighted Averages for Several Rows


I'm building a tracker that will record scores for different regions that will receive inputs once a quarter. I need to roll each region up to provide a single score for each quarter. Since each region might have a lot or very few projects underway in a quarter I need to weight the overall average score to reflect that.

Simply using Summarise in a report won't work, since that will just give me a simple average of the scores (at least I think that's the case, not 100% sure). And I can't seem to work out if a Parent/Child record structure might work.

Does anyone have any advice?