Okay, your bromine levels of 0.5 to 1.5 ppm are lower than the recommended levels of 4 to 6 ppm. Since you don't have access to sodium bromide to build a bromide bank, and have crushed 6 of the 1" BCDMH tabs, you may not have as much as 50 ppm bromide bank, but you will have some bromide bank, maybe at least 10 to 15 ppm, and that will increase as you dissolve additional tabs in the floater. If you have 15 ppm of sodium bromide in the water, you can increase your bromine level (hydrobromous acid) to 15 ppm by adding the right amount of MPS oxidizer, which will convert the sodium bromide into hydrobromous acid almost instantly.
You can use pool math to figure out how much MPS to add. Pool math does not have a setting for bromine, and it does not have an option for MPS oxidizer. However, you can use chlorine units and convert back and forth between chlorine and bromine. I have found by experimentation that 1 teaspoon of 43% MPS (this is the standard in the USA) is roughly equivalent to 1/2 oz of 8.25% bleach.
So you can do an experiment to test this out. I don't see the volume of your spa, so you will have to set the pool math calculator for your spa volume first. Now, take a bromine level with your test kit. Be sure to let the sample water cool down below 90 deg F before you test. Let's say your level is 1.5 ppm and you want to test raising it to 6.0 ppm. Convert to chlorine equivalent by dividing by 2.25. 1.5 ppm bromine = 0.666 ppm chlorine. Put that in pool math in the "Now" column for FC. 6.0 ppm bromine = 2.66 ppm chlorine. Put that in pool math in the "Target" column for FC. On the top line, put 8.25% for the strength of the bleach. Let's say your spa is exactly 400 gallons. The result will read "Add 1.2 oz of 8.25% weight bleach." Now convert that measure of bleach to tsp of MPS. 1.2 oz would be roughly 2-1/2 teaspoons of MPS. Put that amount of MPS in the spa and turn on the jets and air to stir the water. Now take another reading with your test kit. If you have an adequate bromide bank, your reading should be somewhere close to 6 ppm. Remember to use your actual water volume and do your own calculation.
I suppose if your kitchen measures are metric, you will have to do some additional conversions.
With the 3 step method, you should add oxidizer (MPS) after each use of the spa. The amount you add will depend on the number of bathers and the time in the tub. Also, adults would produce more bather waste (usually) than children. The idea is to exactly neutralize the bather waste added to the tub, so that the bromine ppm after soaking would be roughly the same as the bromine ppm before soaking. The floater should be set then to keep the bromine level as close to the same as possible between soaks.
I found that for my wife and I, soaking in 215 gallons of water at 101 deg F for 30 minutes, I need to add 1 to 1-1/2 oz of 8.25% bleach, or 2 teaspoons to 1 tablespoon of MPS, to neutralize the bather waste.
Note: About minimum sanitizer levels. The Chlorine/CYA Chart indicates that with no CYA in the water, the minimum free chlorine level for safe sanitization is very low. However, there is a footnote that states: "A minimum FC level is needed as a "reserve" for usage so in practice at least 2 ppm FC is required even at low CYA levels. The table above shows the amount needed for disinfecting chlorine for equivalent killing power (rates), but does not take into account the amount needed in reserve to prevent getting used up as this varies by pool." Now, this information is for pools and not spas, and for chlorine and not bromine. However, a minimum of 2 ppm of FC would be equivalent to 5 ppm of bromine. So if pools and spas are even roughly equivalent, it would seem that 5 ppm bromine would be a safe minimum. It's probably a little lower than that since the recommendation is for 4 to 6 ppm, but it gives you an idea of how inadequate 0.5 to 1.5 ppm bromine would be.
My bromine levels are as low as 0.5-1.5, is that okay or too low? I got 3 small children using the spa pretty much every day after day care, so I don't want too much bromine if I don't have to.