Spelling Rules

[kutsuwamushi.livejournal.com]

I hate my textbook sometimes. I can't find a single coherent explanation of either of these spelling rules - just bits of them here and there. This is what I've put together:

After the letters г, к, х, ж ш, щ and ц, the letter ы is always written as и.
After the letters ш, ж, ч, щ, and ц, the letter о is written as е if it's not stressed.

Am I close? I'd really like to have these right before I tattoo them to my forehead.