@Wann****
that would be incorrect and
@El***
makes an excellent point, this is the very FIRST yearly extension that the O/P would be applying for so the "balance can't go below 400K baht would not apply because they don't HAVE an extension now, but are here on an entry stamp. The seasoning requirements of the 800K baht in the bank 2 months before the extension is applied for, keeping it in the bank for 3 months after the extension is issued AND then not letting the balance go below 400K baht the rest of the year would only come into effect AFTER they get this first extension.
To apply for that using banked money they just show the money in the account for 2 months before they apply for the extension.