I use Chlor-Tab 500 tablets for all my sterilising, which are also Sodium Dichloroisocyanurate. Easy to adjust the strength by the volume of water, and rinsing removes any residual smell. They seem to work very well for me, I've used them for years and never had any problems...
The '500' comes from one tablet in 1 litre of water making a 500ppm solution, so one tablet in a gallon makes a 100ppm solution. Each of these tablets is about 3g, so max is right - your 400mg tablets might be a bit weedy. Does the pack tell you the ppm of the recommended solution?
The tablets were supposed to be a direct replacement for Sodium Hypochlorite, but more effective, releasing more free Chlorine into solution.