Here is a good range, however, it is best to find what your tank is normally at and try to maintain it there. A good baseline is what your salt naturally mixes at
Nitrates: 5-20
Phosphates: .05-.3
Calcium: 375-450
Alkalinity: 7-11
Magnesium: 1250-1350
Salinity: 1.025-1.027
Iodine: .04-.06
I've seen thriving tanks where their numbers are no where close to these, so again it is important to find what works for your tank and go from there.