Re 27% success of the two-vowel pronunciation rule:
Rules ought to work with very few exceptions, or they aren’t rules. I didn’t work out all of the combinations like you did, just enough to see it’s not a rule for teaching.
Thanks for carrying it all the way through. See, “ou” prounouces as “oo”, not as a “OH”, but you got that!
I didn’t work out the combinations myself. I got out my Spell to Write and Read phonogram cards. The program is genius. I used it to teach my children to read and spell. There are more advanced sounds for some of those combinations as well but I just stuck with the basics.
Here is Wanda Sanseri’s speech to the Senate. Excellent reading!
http://www.bhibooks.net/f/Senate_Speech.pdf