My tap water is 8dKh and 11dGh, so I figure with 50% water changes at least every 14 days, usually 7-10 days, I have plenty of calcium and magnesium, so I don't dose any epsom salts.
My 75 tends to have plenty of nitrates, and phosphates are usually within balance, so much of my dosing is K and traces (including Iron). I try to dose traces every 2-3 days, alternating a little iron on the other days. After a W/C I dose about 15ppm K, and then midway to next W/C I try to dose another 5-10ppm of K, kind of going off how the plants look and how much they have grown in the last 4 days.
In other words, kind of a lazy estimative index, aiming for the targeted range of macros, and assuming traces are all good.