Every week I make sure to test KH, Ca, and Mg. If those test are not the same or close to as the week prior I will test KH throughout that next week to ensure stability as it's the easiest test.
I test Iodine, Potassium, and Iron every 2 weeks and dose these up accordingly.
I strive for
~475Ca
1500 Mg
4 Meq/L KH
Do not care about nitrate/phosphate. Not a problem for me.