Homebrewing - Beer, Mead, Wine, Cider

I have been struggling to stabilise things in my last few brewing attempts. I had been using a combination stabiliser (sorbate and sulfite) from the department store Boyes. It doesn't seem to work.

I know have sorbate powder and I already had campden tablets. I am wondering how you dose them correctly. From what I understand it's dependant on the ABV and the pH. Is there an easy way to calculate this? I take it there is no easy and cheap way to do a free SO2 test.

I am begging to think buying a sous vide and doing pasteurization is easier and more reliable at this point.