If it's any consolation, I can't keep up with it either (even with CPE).  I have to look it up every time.  I've thought of making a flow chart but then there are certain situations where nobody is quite sure what the rules are.  The IRS seems to think that the 10-year rule added to old rules, other professionals think it was Congress' intent that the 10-year rule apply instead of the old rules.

Vanguard has a tool here that you might try:

https://inherited-rmd-calculator.web.vanguard.com/

If you have TheTaxBook or QuickFinder they might be helpful.  Otherwise you're left trying to sift through Pub 590B:

https://www.irs.gov/pub/irs-pdf/p590b.pdf

Rick