What are the conditions regarding the minimum balance for a non-O multi-entry visa in Thailand?

Yes you are right that the 400k is the minimum threshold for the 12 month extension period.

@Keith ********

if you are you on a 90 day Non-O and a year extension w/a re-entry permit that you got from the immigration office where you live by showing proof of funds the seasoning for banked money method is like this:

800K baht in a thai bank account for 2 months BEFORE you apply for the year extension.

Then the 800K baht has to stay in the account for 3 months AFTER the extension was granted

and

after that the balance can't go below 400K baht the rest of the year <- until the following year you get it back up to 800k baht 2 months before you apply for the next year extension