Barry,
By the way, 3 fluid ounces of 6% bleach in 365 gallons would be about 4 ppm FC. I'm not sure why your spreadsheet lists things like "4 oz bleach (8FC)" and "1.5 oz bleach (7FC)" and "3 oz bleach (8FC)". The (*FC) is just the target I was shooting for. In the list below, I calculate the presumed chlorine demand or usage.
The CC shouldn't be persisting so is probably coming from something other than your sweat/urine. I also noticed you were adding enzymes and though that might help break down organics, they clearly haven't been helping to reduce CC and they just get broken down eventually with chlorine so increase chlorine demand. That makes sense. The KISS principle of keeping it simple applies here -- the less you add to the spa the better.
By any chance does your spa have an ozonator? YES If it did, then that could account for the chlorine demand when not soaking. However, your pH would probably be rising more if you had an ozonator. Is your spa covered when not in use? YES Is this a new spa or your first fill of water in it? Less than a year old - 2 re-fills so far. Has the spa ever been super-chlorinated? NO When you say "soak", how many people and for how long? 2 people 45 mins avg. Is there anything else in the system like Nature2 or other ionizer? Have a Spa Frog cartridge in filter well.
Personally, unless you smell the CC, I wouldn't worry about it until the next time you do a refill. Really no smell at all even with CC values over 1.
What is interesting is that when you were shocking with bleach, the FC was dropping fairly rapidly, but you also had enzymes at that time and the shocking probably just got rid of the enzymes. Unless you believe you really need to use either MPS or enzymes, you could use neither and see if the chlorine demand stabilizes. Sounds like a plan - I'm all for less additives. Since even use of Dichlor got the CC up fairly rapidly while the bleach seems to have taken longer, I'm thinking that there is something in your water that combines more slowly with chlorine and stays persistent and my best guess would be the enzymes except that you saw this early on with Dichlor and didn't record any use of enzymes then.
Also, since this is water from a cistern, then the next time you refill the tub you should 1) use a water filter (softener) to remove calcium and iron, etc. and 2) shock with LOTS of chlorine -- 20-30 ppm FC and if you use Dichlor for this then that will get you the CYA you need pretty much in one shot. If you instead use a better water source, then you shouldn't need to go through this hassle. Thanks!
Richard