The logical approach would be to practice attacks that require precision movement. There is, however, more than one type of precision I find. There's things like Hourai Jewel or Honest Man's Death which require you to just tap at proper (very small) length consistently. On the other hand, there are attacks that require precision with unfocused movement and stopping at the right place such as Xu-fu. Then there is precision with circular movement like Imperishable Shooting. There are obviously also attacks with more than one of those. Ex-Keine's second spell card for example requires very precise movement circularly as well as proper tap movement lengths, if you want to do it well.
Basically, supergrazing requires character control and if you want to improve your precision, learning some would help, probably. There are of course other things that require character control to be done properly, such as cancels in Imperishable Night or some positioning in games like SA and EoSD. However, nothing will be a guaranteed improvement. Skill often doesn't transfer, not to mention character speeds are different between the games, unless you play Reimu/Marisa in every game. There is also the thing that even if you can do the grazing, it doesn't mean you're doing them well, which again would not help much; but doing them well requires time and practice that may not be worth the time you are investing.
I'm inclined to say that as your goals become more difficult and reach higher levels, your precision will improve naturally and trying to force the issue might just distract you from practicing other things just as important that you are not good at yet. But if you do want to try the grazing bullshit I'm spouting here, then PCB and IN are probably the best games for that. A lot of variety in those two.
PS: setting games to 90 fps isn't helping because the character also moves 50% faster, so when you go back you still don't really move the way you want.
