Yes, 1 part-per-million (ppm) = 1 milligram per liter (mg/L).
You should use
PoolMath to go from desired ppm changes to weight of product in grams because the weight used for mg/L or ppm is NOT the same as the weight of the product. For chlorine, for example, it's the weight of molecular chlorine (Cl
2) while for Total Alkalinity (TA) and Calcium Hardness (CH) it's the weight of calcium carbonate (CaCO
3). This has nothing to do with us, but is how the pool/spa industry defines these things.
So if I set PoolMath "Units" to "Metric" and put in 875 liters for "Size" then if on the "FC" line I put in 0 for "Now" and 10 for "Target" and select "dichlor" in the pulldown menu, I get 16 grams. So every ppm FC in your spa is 1.6 grams of dichlor. If I put in 8.25 for the % for bleach, then I see it is 101 ml so every ppm FC in your spa is 10.1 ml of 8.25% bleach.
Having an ozonator will be good in roughly cutting in half the amount of chlorine you would otherwise need after each soak, but ozone also reacts with chlorine so that in between soaks your chlorine usage will be roughly doubled. So an ozonator works best in a chlorine spa when you use the spa frequently which fortunately you will be doing.
Two people soaking for 30 minutes is one person-hour of soaking which in a hot (40ºF) spa with no ozonator would normally need roughly 9 grams of chlorine (Cl
2) which is 16 grams of Dichlor or 104 ml of 8.25% bleach. In your 875 liter spa it's 10.3 ppm (mg/L) Free Chlorine (FC).
So with your ozonator you should be able to add roughly half these amounts after each soak to get you to the next day, so around 8 grams of Dichlor or 50 ml of 8.25% bleach. In between soaks your FC may drop in half each day due to the ozonator. The real rule to follow is to add enough chlorine after your soak so that you end up with around 1-2 ppm FC for the start of your next soak. You'll need to fine tune how much you add to figure out what that would be, but once you figure this out for your spa it should stay fairly consistent.
The difficulty will come if you ever want to take a vacation or not use the spa for more than a couple of days. You could get lucky where enough ozone gets into the water to keep the spa OK, but we've seen plenty of reports of spas with ozonators that turn bad if the chlorine level drops to zero. If you can figure out a way of temporarily disconnecting your ozonator, then that would have the chlorine last at least twice as long and if you lowered the water temperature you should easily be able to go away for a week and have the chlorine last if you boost it (say to 10 ppm FC) when you leave. For longer than that, you'd probably need someone to add chlorine once a week. You should find out what kind of chlorine bleach source you will be using so that you know its concentration -- I assumed 8.25% but that may not be what you get.
So now that we've talked about chlorine, we need to talk about adjusting your other water parameters. Once you get a proper test kit then post the pH, Total Alkalinity (TA), and Calcium Hardness (CH) numbers so we can go through how to adjust them as needed. You should also see from the threads I referenced earlier if you can get boric acid. 50 ppm of it in your spa will be used for additional pH buffering. Using PoolMath entering 0 for Now and 50 for Target, that would be 250 grams for your 875 liter spa. As was mentioned in an earlier link, you can get boric acid from the UK via eBay (see
this link for a listing) or you can get it and other U.S. items shipped to you from mvoorn whom you PM'd. If you are interested in why it is so hard to get in Europe and about its toxicity you can read
Are Borates Safe to Use? where basically the EU is regulating it on the basis of its concentrated form rather than on its concentration on use. You won't be using the powder in a manner where you would inhale or ingest it and it will be very diluted in the water which you aren't going to drink in large quantities for a lifetime (and it is not volatile nor does it absorb through the skin).
If you follow the Dichlor-then-bleach method, you should be able to go at least twice as long between water changes compared to using Dichlor-only. Though there is some work to get the spa set up after a refill of water, it's pretty easy after that just adding Dichlor after each soak until you build up CYA, then switch to adding bleach after each soak. Once a month you'll use Dichlor for one day (i.e. for one soak) to restore CYA that slowly gets broken down by chlorine. That's it for disinfection. The only other periodic adjustment would be for pH, but we can go over that once you get a test kit and numbers. Once you get your TA lowered and add the boric acid, the pH may be pretty stable. If it rises too much, you add some acid to lower it.
