Today, I figured out how to simplify the explanation of the net-flow rate. It makes no real difference either in calculations or in the complexity of the spreadsheet, but it'll reduce the difficulty of explaining it to non-demographers.
Here's the gist: the increase in enrollment for grades x on up between any two points in time is equal to
The last two terms can be collected as residual net flow, can be estimated, and then from cumulative raw numbers, you can infer grade-specific raw net-flow numbers (and thus rates).