@Jost *******
if you're legally working here with a work permit for a company and your salary is over 40K baht a month you can use that to meet the proof of funds.
OR
if you work abroad and want to use your salary as proof of funds (and your embassy doesn't issue an affidavit of income from abroad notary letter) you transfer in to a thai bank account in your name only a minimum of 40K baht a month each month every month for the previous 12 months before you apply for your yearly extension.
OR
You bank the 400K baht in a thai bank account in your name only for 2 months before you apply.
Those are the only 3 ways to meet the proof of funds