Limit Rollup Hierarchy

We're are trying to use a couple numeric custom rollup fields to count the number of tasks that are tagged with a value of 1. We are doing this so we can show the different field counts on the same charts using multi-dimensional grouping.

Our problem is we only need to count down to the tasks that roll directly to the milestones. Which means if any sub tasks are added we can't assign a 1 since it is no longer the lowest level. Is there a way to limit what the rollup thinks is the lowest level? Thanks