its any parameter a salt mix will mix to, since all salt mix brands show they can be used on reefs. Any degree of shifting difference per mix, allowed. some are high in mag, some low etc.
the best water params are high degree of suspended feed that isn't rotten, and its varied, some of it living if possible. export/physical current/manual cleaning or animal cleaning of surfaces to unstick all the waste detritus off rocks, and then some repeating loop where feed never suffers, and export occurs before rot in the water and subsequent waste. its becoming more evident in time that no one set of params works across tanks, since feed consistency ranges. High alk can bleach out low feed systems, so anyone posting a higher alk may not carry over to another setup very well.
the ideal param is a set of physical conditions factoring nutrient availability, its chemically anything a salt mix will mix up to. to not have a set param goal allows for flexibility in design and makes us responsible to drive our own reef's mass with action, not pin it on chemical reasons we haven't attained the best coral growth. sticking pretty close to your original measures whatever they may be, whether by water changes or dosing, is the right set of params. we can find sps growth in all params a salt will mix to. The best pico Ive seen is using instant ocean for 8 years.