brackish water is roughly 4 tablespoons of marine salt per gallon of water. That will give you a salinity/S.G. (i never remember which is what....) of 1.010....you need to check with your hydrometer, b/c different mineral compositions in your water may give you different readings with those porportions. A good hydrometer is made by MarineLand....it's a plastic 'box' that has a 'dial' on it....about twelve bucks at petco. easy to use, easy to read.
I think it is called 'instant ocean hydrometer' or somekthing like that.
I don't think a Betta can handle that much salt.
marine salt is totally different from aquarium salt though. if you want to do brackish, use marine.....and acclimate your fish to the brackish water verrrrry slowly as not to shock their systems. start out at 1.001 S.G., ....and bump it up .001 every 3-4 days or so till you get to 1.010 S.G.
Froggyfox is right...aquarium salt is mainly used for medicinal purposes....and, IME, I don't find it to help much, aside from being a good treatment for ich.
If you need any pointers on acclimiating a fish from FW to brackish, I can help you....i have about another week or so before the puffers i am introducing to brackish will be going to a fully brackish system.....they were kept, sadly, in a FW system.....