This is the exact reason behind the dichlor then bleach method as mentioned in this article
If you have a question about any information within this Sticky, please start a new thread for yourself where you will receive very quick responses to your specific questions. Thank you! Introduction There are two main things to consider when maintaining your hot tub water, Water Balance and...
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Time to switch to liquid chlorine- always follow the
FC/CYA Levels
Cya of 40 isn’t a crime lol

but above 50 makes it hard to keep things sanitary while also maintaining ph properly (since the ph test is invalid at fc levels above 10ppm)
The cya in a hot tub gets depleted slightly over time because of the hot water so I try to replenish it with a dichlor dose every couple weeks or so. Don’t sweat that it’s 40ppm - just chlorinate accordingly
I'm going to add something totally un-necessary, kinda like you having a calculator and a slide rule (useful when you have a dead battery).
The calculations for adding CHL are just math, like percentages but for such small needed quantity, you use ppm, parts per million, instead. So if you divide by 100 you get parts of a dollar, cents (per cent), ppm is just divide by a million.
So, first, you are adding ounces but you have gallons in the tub. So make it ounces, apples to apples you know. 515 gallons (my tub), 128 ounces in a gallon:
515 X 128 = 65,920 ounces of water
So now, you want to add some CHL. What you add is ppm. One ppm is one millionth of a part of that 65,920 ounces, or:
65,920 / 1,000,000 = .06592 ounces
This means for every .06592 ounces of CHL you get in there it should read 1 ppm on your test kit.
So, bleach used to be mostly 5.25%, but I'm now seeing it left off the labels when it's bargain unscented stuff and it seems to be anywhere from an intentional 3 - 3.5% (Kroger comes to mind), to 5 or 6%.
You expressed in interest in 12.5%? I recommend 10% and will base the rest of the calculation on that (because it takes very little, you will be measuring even smaller quantity with more potent product. So this actually gets easier. Dose in ounces, compute when you need less than an ounce is simply proportionate --- meaning if an ounce added 3 ppm and you need one ppm, just add a third of an ounce.
Enough over-explaining, sorry. So an ounce of bleach or whatever you buy as chlorinator that really is the same thing but stronger, if it's 10%, an ounce has 10% of CHL, or 1/10 ounce (.1 ounce CHL) per 1 ounce solution. 12.5% solution of course has .125% CHL (.125 ounce CHL) per 1 ounce solution.
So that ounce is how many ppm of the water in the tub? For each ounce of CHL you get .06592 ppm CHL, divide:
.1 / .06592 = 1.52 ppm CHL for every ounce of 10% solution you add to a 515 gallon tub.
(.125 / .06592 = 1.9 ppm CHL for every ounce of 12.5% solution you add to a 515 gallon tub, if you must

)
I use a Taylor scale that also measures in grams, making the readout higher makes it easier to stop pouring, gives me better resolution, and may be more accurate since the scale is expanded. An ounce of solution is 28.3 grams (just multiply ounces by 28.3).
I'm adding 3.15 ppm an ounce for my tub with that 10% solution, but the pool math calculator agrees with the above 1.52 ounces. Still, when all is said and done, your solution will vary in concentration, so you can just use this to add a few ppm, run a couple minutes to mix, and retest with a drop tester and adjust accordingly. Now that the container is almost empty it's 7.5% BC I needed more to hit my target number. Kinda makes regular testing a thing, and an accurate DPD kit seems prudent.
One last note bc I'm very long winded ... I use a medicine dosing cup. When I used low % bargain branded bleach I used a plastic cup. My scale has a tare button to cancel out the container but the dosing cup I just read off it. Of course pouring a scant 1/2 ounce from a full gallon bottle isn't fun, so I just used the old cup and added an ounce or two and repoured, then put the rest back in the jug. I'm cheap. That first (of 4) gallons lasted me about 5 months before I purged (demand dropped after using ahhSome! to about half what it was).