Assuming the worst case scenario - If the entire 800,000 was remitted with no overseas taxes paid on it, and it was deemed to all be assessable income, then your allowances would play the biggest part.
For myself, being under 65 and married (but on a retirement extension), and claiming the maximum THB 25,000 for my health insurance premium, my tax would be THB 50,750 in such a scenario. Not too great, but I would still be looking at ways of (legally) minimising it.
(Spreadsheet from PWC, who used to file my taxes when employed here).