You have to look into your joist spacing from underneath if you want a calculation or a formula to work with. Then your direction of install (or "preferred" direction of install would be a better choice of words) will then dictate what you need for a subfloor.

A double layer of subfloor measuring 3/4" deep is going to be your "starting point". If you go thicker, you are simply adding a sturdier underlayment (which is a good thing).

Your direction of install, along with your joist direction and your joist spacing will dictate everything else. A well trained flooring professional can have a look at the direction and spread of the joists and the current thickness of the underlay. They s/he will figure out "deflection rating" required for your "preferred direction of install" and do the calculations from there.

Have fun. Ask your professional installer to check the joists and to get back to you with recommendations. That's when you will find out the quality of the professional involved.