Thanks for your input. The 60% ideal makes sense in general. Now you have jogged my memory . . . ALEKS actually permits moving onto the next math course based on 85% mastery although by default, they have you working/testing through 100% since it's assessed by topics represented as pie slices.

It may partly be that my standards are too high based on my own level of mastery, but it seems to me that DS hasn't achieved complete mastery.