It's
HERE.
It was written by/for people with great familiarity with TFT and no difficulty doing math.
I'm still using it in my new TFT play, but I think:
1) I think I'll eventually settle on a guideline system somewhat like you suggest (though mine would be more complex and I'm waiting for an inspiration to arrive as to how it will work exactly).
2) I could now make a better formula, but it'd be more complex and I'd probably make a program to do the needed math. :-)