I do 7 drops per gallon (based on SRC's recommendation way back when this topic started). It seems to work fine.
If I use more than that, I get a yucky slimy film on the top of the water that reminds me of when I used to go boating with my dad and he spilled gas in the water.
One other thing I have read in all my months of researching how to fix up little ailing guys or girls... one website stated that in some cases, if you use too much stress coat, it makes a really thick film on the surface, and since bettas use the surface to breathe, it can cause them issues with being able to take in really good solid breaths.
Just my 2 cents worth - the only time I ever use more than 7 drops per gallon is when I'm treating for columnaris, or the fish in that tank has had some sort of trauma, ripped fins or body damage. It helps immensely with the slime coat..