Are there easy ways to remember the 30° and 90° rules?

Yes. Here’s some poetry:

30° rule:

If you let one finger stay,
The other finger points the way.

90° rule:

When the ball has stun,
this is something you should not shun:
Point your finger, and the cue ball will follow the thumb.
If you do this, nobody will think you are dumb.

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