I have been running a calcium reactor for over 20 years, long before there were controllers. So you can run a calcium reactor by counting bubbles of CO2 and watching the effluent drip rate. If you plan on having a lot of SPS, calcium reactor is really the best way to go, assuming you are talking about SPS frags. The up front cost is higher, but in the long run, they are cheaper to run than adding 2 part frequently, especially as your frags grow and consume more and more calcium. If you are handy, you can even build a reactor yourself. There's no rocket science to them. It is just a chamber that mixes CO2 rich water with calcium carbonate substrate.
If you don't want to go the calcium reactor route, you can always add Kalkwasser. There are plenty of write-ups about adding kalk to your top off water to raise your calcium level that way. But really, those are your only 3 options. Calcium reactor, 2 part, or Kalk.