How often are you doing water changes, how old is your system, and do you see a decrease in Kh, Cal, and Mag between water changes? Simple method is to do a water change and dose to get your levels to where you want them, you may not need to do this depending on the salt mix you use. Let your system run for 24 hours then test again. If you testing shows a decline then this is how much your tank is using in a 24 hour period. Do a manual dose again and test a couple hours after to make sure your back to where you want then run for 24 hours and test again. If you again have a decline you now have a 2 day trend and so you now know your tank consumption. You can continue to manual dose or set up dosing pumps to make up the difference.
If your tank is new and has few corals and your are good about water changes weekly then it will be a little bit before you will need to worry about dosing pumps. I would just continue to test, monitor and adjust until you see larger consumption. I know people that manual dose 50ml plus each day because they don't want to worry about mechanical failure of dosing pumps. Hope this helps and makes sense.
