regarding the need to have 400,000 baht in a thai bank account for 3 months in order to get a one year marriage visa extension, how is the 3 months calculated?

Do they go back 90 days from the date you go to immigration to get the extension or do they use some other method to determine this?

3 Calender months from the day you apply.

E.G. Apply on 16th November they will look back to 16th August.

For the first application it is 2 months.

The 3 month period is calendar months calculated backwards from the date you apply to renew your extension of stay (2 months for first application).