When I was using bromine I used the little packets of bromine start up. if I remember correctly they were for 200 gallons of water so I added two for a 400 gallon tub. Do what it says on the start up package.
Bromine was always confusing to me because there is always a bromine bank in your tub and when bromine kills something it converts and then when you shock it converts back, or something like that. Chlorine seemed simpler as it gets used once and is gone.