How does the income letter method work for extending a Non-O visa based on marriage in Thailand?

Extension based on marriage. Income letter method.

How do the policy work regarding the age of the income?

I have a taxation I can have my embassy to write the income letter based on. That is for the 2017 income. From this summer the tax agency will have a new result based on income from 2018.

Will the income letter I can have now be valid for a year long extension. Or.. Do I need to wait for the new, this summer or do they make a shorter extension based of those facts?

I do have the Non O ME based on marriage valid until late summer at the moment. So no panic. I was a bit bored about the borderbounce and thinking that I might be able to use the old income to avoid more borderbounces.