By the chart on this forum I assume you mean
this Chlorine/CYA Chart. The chart in the Pool School (
Chlorine / CYA Chart) is a simplification, mostly rounding, of the chart in this forum. They are not inconsistent. It's simplest and easiest to follow the Pool School chart numbers, but that's up to you.
As for safety, we generally say that you can swim up to the SLAM (shock level and maintain) level though you will notice the chlorine more at that level. The active chlorine level in the pool at shock level is roughly the same as a pool with around 0.6 ppm FC and no CYA so less than the amount of chlorine found in indoor and some outdoor commercial/public pools. The high FC level when CYA is present is only an issue if one were to drink very large quantities of pool water regularly. The EPA limit of 4 ppm FC is for drinking water at 2 liters per day every day for a lifetime.
Your example of 6 ppm FC with 40 ppm CYA is not in any chart I could find. Did you mean 50 ppm CYA? Then the target is 6 ppm FC in that case (minimum is 4 ppm FC). 6 ppm FC with 50 ppm CYA has the same active chlorine level as only 0.1 ppm FC with no CYA. Over 98% of the chlorine is bound to CYA and is effectively inactive, doesn't react significantly, doesn't outgas, has no skin absorption, etc. This is why you shouldn't be looking at solely the FC number in isolation to determine the effectiveness of chlorine or whether it is safe. It is the FC/CYA ratio that is most relevant as that relates to the active chlorine level that does the actual disinfection killing pathogens, prevention of algae growth, oxidation of bather waste, oxidation of swimsuits, skin and hair, and outgassing. The recommended level we set is that which is sufficient to prevent green and black algae growth regardless of algae nutrient (phosphate, nitrate) level. It kills pathogens very quickly, but is not so high as to be harsh or noticeable.
