Can be a real pain Peter,
Wont go at any price if you have to much shim in there and it doesn't take much to be too much, the circlip being circular in section you might have to allow a bit more than half of that circle to be outside the groove, therefore the top of the bush to be a little below the groove - does that make sense??
Also I bought two new circlips from a well known BSA dealer and official that were two big for the groove, fortunately I did not on that occasion loose a circlip to the gods of free flight