How does the monthly proof of income work for retirement visa renewals in Thailand?

I am an American and have had a retirement visa since 2011 – I must renew my extension of stay on 27 Dec.

I have several questions concerning monthly proof of income. Does the amount have to be 65,000 baht or more or can it be 50,000 baht one month and 80,000 the next?

I unwittingly put several thousand dollars in my Thai bank account on Dec 24 – does that count towards year 2020?

What financial products are allowed or excluded.

Does the Thai government publish any unambiguous rules on this subject that can be referred to at a later date if needed? My local Immigration office is Jomtian and very busy and understaffed. Technical information provided by the staff related to the above may prove to be inaccurate eight months from now. I certainly don’t want to start the process all over again if I don’t have to.