Hi Crunchy,
The difference in the TA and the CH has to do with the Calcite Saturation Index (CSI). CSI is a measure of water balance. If its too high, it can indicate you are at risk calcium scaling, and if its too low, it can indicate you are at risk of the plaster slowly dissolving away.
TA buffers pH rise. If the pH rises too quickly and you have to add a lot of Muriatic Acid to bring it back down, then you can lower the TA to help out with that rapid rise and not have to add acid as often. Many of us have our TA all the way down to even 50 to control pH rise.
However, with the lower TA, you need to raise the CH a little bit to keep the CSI within limits. The science behind the CSI gets very involved and frankly, few of us understand it in large detail, but there are some here that do. You can probably read more about the science by searching "calculating CSI" or something to that effect. There is a search box at the top right of the page under the Pool School button.
Just below the Temperature row in Pool Math, is the CSI calculator. You can play around with it and see how changing your water parameters effects the CSI.
The TFP recommendation for FC, is to prevent algae as well as kill pathogens. Chlorine will kill agae and prevent it from reocurring if at the correct level.
If you keep your FC at 7.5% minimum to 11% of the cya level, that will prevent algae.
The Pool industry recommendation of 2, is for sanitation only, and then use algaecide and phosphate remover to control algae. The reason they do that is so they can sell more stuff for lots of money. The problem tough, is that it is expensive to go that route and honestly, it isnt nearly as effective as keeping your FC/CYA ratio per TFP recommendation.
Here is the science behind the FC/CYA ratio that we use.
Pool Water Chemistry
The CYA recommendation of 70 is to lessen burn off of FC by UV from the sun. Doing so protects your FC a little longer and doesnt require you to run your SWG and pump quite as long, or at a higher ouput of the SWG. So, ultimately, this saves wear and tear on your equipment, saves on electricty and squeezes a little more life out of the SWG cell.
TFP does not recommend a salt level of 4200. I dont know where might have gotten that information. We recommend whatever the equipment manufacturer recommendation for salt level is. For SWG systems here in the US, that is usually somewhere from 3000 - 3400 ish, but there are some that recommend a higher salt level. Here is a link in TFP Pool School about swg levels. When in doubt, the information contained in Pool School always takes precedence as the official TFP recommendation.
Pool School - Water Balance for SWGs
I hope this helps a little.