The granular chemical you have will work as it is basically dichlor (a source of oxidizing chlorine) and sodium bromide (a source of bromide ions). When you add it to water, the chlorine will react with the bromide to convert it from its inactive state (-1 oxidation state) to its active bromine form (+1 oxidation state). The chlorine simple gets reduced to chloride salt in the process. As is with most spa chemicals, its a costly version of the much cheaper method of buying sodium bromide granules online (it's a cheap, harmless salt) and using regular store bleach to activate it. All you need to do is add enough sodium bromide to create a 30ppm bank of bromide ions and then use bleach to activate it. For all intents and purposes, you can simply define what bromine level you want, then divide it 2.25 and that's the amount of FC you need to add to get there. You can then add that amount of bleach and then retest to see how close you got to the bromine level you wanted. It's a bit of trial error in the beginning to circle in on what you need to add but you'll eventually get to know how your tub reacts and after a while it will be easy.
You can also use dichlor granules in the same way but it does add CYA to your water which won't affect the bromine but it can change the pH/TA balance a little bit. Dichlor is also mildly acidic so it will lower pH with use. Bleach is relatively neutral in terms of pH and doesn't add anything to your tub water.
In order to get 30ppm Br- bank in 420 gallons of water, you need about 61.3 grams or 2.2 ounces of sodium bromide. It's really not super necessary to be that accurate as the bromide bank is just there to act as a reserve of bromide ions so it could be 30, 40, 50ppm or whatever is easiest to add. 30ppm just ensures you have enough bromide in the water so that if you want to shock the tub, you can get up to the 20ppm bromine level easily. Bromide doesn't go away so you should only need to create the bromide bank once per fill and then it's good to go.
Stay away from bromine tablets or reserve them for use only when you'll be away from the tub for significant periods of time. Brominating tablets tend to dissolve too quickly and they are acidic. So there tends to be wilder fluctuations in bromine levels as well as pH issues. They also add DMH (dimethyl hydantoin) to the water which acts a lot like CYA and, at high enough levels, can reduce the effective strength of the bromine leading to sanitation issues.