However legally I am not allowed to exceed 5 mg/l as a private pool. Does that then mean I am forced to have the CyA at 30?!
I don't know about pool regulations in Belgium, usually only chemical levels in public pools are regulated as far as I know.
If that is the case, then you need to choose a CYA level that allows you to stay within the legal FC limits. As you said, either by choosing CYA 30 to have most of the target range available. Or go 40 and keep FC between min and target, but that will require more frequent testing and dosing to ensure not to drop too low.
Lower CYA in Belgium is probably fine, don't forget that the levels recommended here are designed for warmer climates. You have to work out what works for you - every pool is different.
Another option is to go Salt Water Chlorinator (SWG). The regular chlorination including constant super chlorination within the SWG cell allows to maintain lower FC levels.
It's a bummer that the pool industry never bothered to understand the chemistry of CYA and Chlorine. They allow FC up to 5 ppm without CYA, but they freak out at FC 7 ppm with CYA. 5ppm without CYA is far, far beyond our SLAM-FC with CYA in terms of active chlorine and aggressiveness towards swimmers and pool equipment. I'd rather swim in a SLAMed pool than in a public indoor pool with FC 4ppm and no CYA - after that my son smells for days like someone poured a bottle of bleach over his head.
The CYA chemistry was understood and published by O'Brien in 1974, but gets ignored by the pool industry. Search for it in the forum search, it is available here if you are interested.
There are two sticky threads from Chem Geek about pool water chemistry in The Deep End forum, if you are interested. Chem Geek is Richard Falk, a physicist who became interested in pool chemistry after running into problems with his pool. He did a lot of work, going through a lot of scientific papers, many of them existed for a long time but kept being ignored, and contributed immensely to the current state of Trouble Free Pool Care. He is not posting anymore here, but he is still active and currently pushing the US regulators to modify their FC regulations to consider FC in conjunction with CYA. But it is a difficult battle.
Edit: Should you decide to go through Chem Geek's threads, then keep in mind that these threads are old and document the development of the current TFP recommended levels. It's important to not draw conclusions too early on when reading these threads. For example, there are early graphs showing FC half life depending on CYA levels. These graphs only consider FC being protected that is in a bond with CYA. Later on, Chem Geek realised that CYA in higher water layers shields even unbound chlorine in lower layers, making higher CYA levels more efficient than shown in these early graphs. The threads are not an edited book, they document the development over many years.
