This has got to be something that people want to see all the time: averages or percentages of sub-totals within a sub-summary part.
Example #1: A sub-summary part contains the total dollar value of sales (sorted by whatever — color, shape, material, purpose, etc.) and also the total number of units sold (sorted by the same whatever), so what's the average income per unit? Well, you'd think that you could just set up a Calculation field = ∑Sales / ∑Units and get the proper result, wouldn't you? But nooooooo!
Example #2: Sales for this year as a percentage of sales from last year for the same whatever (color, shape, etc.). Does ∑Sales2017 / ∑Sales2016 give you the proper percentage? Nooooooo!
FileMaker should create a type of summary function that works on other summary functions to produce numerically valid results within a sub-summary. We know what the components of the calculation are (they're the summary results we've already defined and which have already been calculated as a result of the sub-summary process), so it should be a snap to just plug them into a formula so they can interact with each other.