The container recommends 1 teaspoon FP per gallon of water once a week for production (which I read as healthy growth). If you water twice a week, you would use 1/2 teaspoon per gallon each time. This is from one of Al's posts:

When plants are growing robustly, I try to fertilize my plants weakly (pun intended) with a half recommended dose of the concentrate at half the suggested intervals. When plants are growing slowly, I fertilize more often with very weak doses. It is important to realize your soil must drain freely and you must water so a fair amount of water drains from your container each time you water to fertilize this way.

If you use FP regularly, you do not need gypsum or Epsom salts.

Thanks Ohiofem.
Since this is my first time using the gritty mix, was unsure of the frequency of watering. From the posts on the subject, it seems that there is no end to the different approaches; some water once per week, some twice a week, some daily, while the underlying goal is "water when the plant needs it". (Won't know that, of course, until I start watering!) If I find myself needing to water more frequently, I'll weaken the strength. Again, thanks for the advice. (You too, Al.)