Fixing the wrong reading first wastes time and chemicals. Here is the sequence that works, with the reasoning for each step.
Water balance is a chain. Get the early links right and the later ones often fall into place. Get them in the wrong order and you can spend a week chasing a reading that would have settled by itself if you had done something else first. The right sequence is alkalinity, then pH, then calcium, then sanitiser.
Total alkalinity is the buffer that holds pH steady. If alkalinity is wrong, pH will not stay where you put it.
Get alkalinity into the 80 to 120 ppm band before doing anything to pH. Add alkalinity increaser slowly, an hour between doses, retesting each time.
Once alkalinity is in band, pH almost always settles into 7.4 to 7.8 on its own. If it does not, then a small pH adjustment is meaningful and stable.
Once alkalinity and pH are stable, test calcium hardness. Aim for 100 to 250 ppm in a hot tub.
Low calcium causes foaming, etching of grout in nearby tiles, and copper pickup from heater elements. High calcium causes scale on the heater and on the waterline.
In hard-water UK areas, calcium is rarely too low. In soft-water postcodes (parts of Scotland, North West, and South West Wales), calcium hardness increaser is essential and often overlooked.
Once alkalinity, pH, and calcium are right, your sanitiser will hold its level and do its job efficiently. Adding chlorine or bromine before fixing the chemistry is wasted product.
Dose to the bathing band (3 to 5 ppm chlorine, 4 to 6 ppm bromine), wait an hour, retest. Adjust if needed.
Finally, test combined chlorine. If above 0.5 ppm, shock with non-chlorine oxidiser. The shock at this stage works far better than it would have done before the chemistry was right.
Common after a long break. Do them one at a time in order, with an hour between each. Resist the urge to dose everything at once. The whole sequence takes a Saturday morning, and the result is a stable tub for weeks.
If it is within 10 ppm of band, yes. If it is 30 ppm out, no. Each subsequent step depends on alkalinity being right.