idk, I think that, on the one hand, you could figure out ratios (like, half the time doing repertoire, a quarter with études, etc etc. (I WOULD ACTUALLY LOOK AT WHAT YOUR WRITING EQUIVALENTS ARE BUT I AM TOO LAZY TO LEAVE GMAIL TO FIND OUT)) and apply those on a smalle scale. But then I guess you'd have to strike a balanced between "working on X for a short but useful period of time" and "oh hell no, five minutes is not enough time to practice anything >:("