Short answer and path to pool nirvana:
Do an OCLT to rule out algae or incoming organic contamination
Keep adjusting the SWC to get the pool to stay within your target band. Start high and dial down in small increments until you stay within your desired range. Don't adjust any quicker than maybe once every few days, and only when it goes over your max or under your min ppm FC. Try to adjust in small increments, e.g. 10% to 20% more or less run time or percentage. If it falls significantly below min, use liquid FC to get back to your midpoint and begin observing effectiveness again.
Long answer, haha...
I think you're doing your math just fine, but a few comments.
I haven't bumped into that calculation of "actual salt" divided by "optimum salt" times "FC production rate". I'm not able to find out how the electrochlor chlorinator works, but it's common for SWGs to adjust current to keep production the same. They have to in order to maintain the spec production level as the cell plates erode. Some adjust voltage. Most people just use the design output for this sort of calculation, but of course everyone is assuming the salt level is within range for the SWC (yours says 5,000 to 6,500), so maybe that factor exists for some chlorinators. I'm still on a quest to learn more about actual salt vs. optimum salt!! It doesn't make any electrical sense to me that it would make a difference, other than when the salt level is seriously below or above the intended range and the design range for the electronics and power supply were exceeded.
Your manual says that the salt light will come on depending on salt level, temperature and mains voltage. I'm the furthest thing from an electronics expert, but I believe that if it varies based on mains voltage, then perhaps the power supply is varying voltage to maintain constant production. If it was varying amps, the electronics could care less about mains voltage. Hopefully an electronics expert reads this and could guess. But given other gibberish in the manual, I don't have a lot of confidence in that statement anyway, and I can't see why their electronics couldn't adjust for mains voltage. Our mains voltage runs 240 to 250 VAC.
This is strictly my own suggestion, but I would give myself a high and wide range (say 5-10 ppm FC) because:
1. the chlorinator output is very high relative to the pool volume
2. high bather load per cubic metre of water has a bigger impact on residual FC
3. 10 ppm FC with 60 ppm CYA is unnoticeable in clean pool water
4. you had a problem in the past due to the cover
When chem additions aren't making sense, sometimes the pool volume is off. You can log your chem addition calcs vs. actual change and see if you're overshooting more often or undershooting more often and adjust the pool volume.
To be sure the pool is not eating chlorine beyond what it needs for UV and bather load, I would do an OCLT.
There's no material difference in SWG production rate at shock level of FC.
Hopefully TFP lands an Aussie SWC expert as good as the TFP experts are on USA chlorinators. It's sure hard to get from the manufacturer's literature! The electrochlor manual starts off with a statement about how the chlorinator works that is entirely wrong, so you don't get a lot of faith in how well they understand their product. In fairness, they stay focused on how to make it work in a pool, which is what counts at the end of the day.
It's on my list to do some measurements on the electrochlor I work with, so some day I might know more. For now with a similar volume pool and that LCD screen almost cooked off by past sunshine, it's all I can do to keep the FC between 2 and 5 (CYA 40)! On top of that, the chlorinator is randomly deciding to run overnight about once a fortnight, so I'm working on that little wrinkle, which pops the FC up to about 8. No problems and swimming is good, but I just wanna fix it!!
That's not much help, mate. But as far as I can tell, your math is certainly sound.
Sorry for long answer but I get the sense you like delving into the details. Be brutally honest if I overdo it!
