Report/Chart Grouping: Count Projects by Current Milestone - how to determine what Milestone a project is currently at?

I have a report help request from an end user. They are trying to create a pie chart of # of projects grouped by current Milestone. Current Milestone = the earliest Milestone task in a Milestone path that's not yet at 100% complete.

Example: There are 3 milestones in a path for each project (milestone 1, milestone 2, milestone 3)

Project 1:

Milestone 1 = 50%

Milestone 2 = 0%

Milestone 3 = 0%

Project is a Milestone 1

Project 2:

Milestone 1 = 100%

Milestone 2 = 0%

Milestone 3 = 0%

Project is at Milestone 2

Project 3:

Milestone 1 = 100%

Milestone 2 = 50%

Milestone 3 = 0%

Project is at Milestone 2

Resulting pie chart:

Anyone know how to accomplish this? Do I start with a Project Report or Task Report? And how do I group and filter accordingly? The logic to determine which Milestone a project is currently at is what I'm struggling with.

Note: in this case, the Milestones are being treated more like gates/phases.

Thanks!