This is just a math problem.
1) Go to
www.cFIREsim.com2) Input your expected ER expenses, and stache, and AA. See what the percent success rate would have been, historically.
3) If not high enough, up the stache amount, and re-run.
4) Repeat step 3 until the percent success rate satisfies you.
(There is also an "investigate" option you can run at step 2 to find how much you need for a given success percentage, that lets you skip steps 3 & 4.)
This is how much to save for ER.
Obviously adjust for any other data you have (common ones include downsizing or moving to lower COL area, pension kicking in, etc. etc.), but it can accommodate all those things when running the numbers to figure out the stache amount.