For someone that never owned a business:
If a dish sells for 60 baht, what is the cost to make that dish for the ingredients, cooking oil etc.
6o baht, minus the cost of the dish, lets say 30 baht cost, leaves you with 30 baht profit
Same thing with drinks however, many drink water these days
Add up all your monthly bills. Rent, electric bill, water bill, trash bill, licensing, salaries for 4 Thais and any other fees
Divide that total cost by 30 (profit for 1 meal). This will be the amount of meals every month you will need to sell just to break even.
Divide that number of meals by the number of days you will stay open per month (20? 30?)
This will give you the average number of customers you will need, every single day you are open just to break even.
When you actually do a business plan and not just dreaming about being a business owner, you can prove if the business is viable or not.
Often it is impossible to get the daily number of customers you need to be successful and to make a salary for yourself
Another thing is restaurants compete with local street food vendors for the same customers. If the street food vendor is selling for 30 to 40 baht per dish and your same dish is 60 baht, guess what? Thai food is something any Thai can do. Remember that.