I have been using something similiar to work on my accuracy. I started with a 1mx1m target at 12 metres and shot sets of 30 tennis balls and recorded the results. Then I moved to a 50cmx50cm target and do the same. As my accuracy reaches a certain desirable point I'll keep reducing the target size.
I like the idea of a target containg several different sized points, like the baelaric targets, but as I walk from my house to my slinging spot its easier to just use a fixed sized target (a square of tape on a concrete wall).
I just got back from 6 weeks holidays but before I left my best practice day was 8 sets of 30 balls at the 50cmx50cm target, hitting it 22,23,24,24,22,24,21 and 25 times on each set. I made a excel spreadsheet to keep track of my progress since last october when I started working on accuracy. It show things like accuracy ratings, average number of hits min/max hits, and makes pretty graphs
I think that session above is ~76% accuracy rating and my goal with this target size was 80% accuracy so I'm getting close, but have been hovering around 65-75% for a few weeks so I might have reached my accuracy cap with the amount of time I have to spend (3-4 times a week, 5-8 sets a session).