Well said. You really need to know where you are, to know where you're going. It can be a PITA for a while, but you'll dial it in. The kicker is, that as your tank grows, it's Ca consumption will change. Ya gotta kinda just stay with it.
What sized tank do you have? what's your total water vol? what's in the tank? LR, corals, fish, inverts?
Most importantly what do you plan to dose? B-Ionic or other 2 part? Kalkwasser? etc...