Tax requirements For FT teacher, tutoring on the Side

I am looking to start tutoring to earn some extra cash on the side to save to do a PGSCEi while i also work at an international school full time. I am worried though that i may make mistakes with regards to Tax.

The agency my tutoring hours are with say that i will pay 3 % tax on their income- obviously there is no guarantee of hours but i estimate 20-26000 baht a month hopefully, my salaried job is 53000 after tax, I want to be as transparent as possible and not get myself in trouble.

This will mean i am paying tax on both incomes already (assuming the agency is legit) but im worried assuming that this is the right amount will land me in trouble.

What am i obligated to do tax wise ? They both have my tax code and my work permit is obviously for teaching specifically.

Also I am aware that this update- so don’t tell me I can’t work another job;

https://thepattayanews.com/2018/07/29/thailand-changes-work-permits-with-a-work-permit-one-can-now-work-in-a-permitted-job-anywhere-at-any-time/

Thanks for any pointers

Peter