Every tank / system will vary by it's unique bio load. Suggestion though.
Do a weekly water change with newly mixed up water. Put whatever weekly addatives you use in now also. Do your cleaning and any scheduled maintainance. Now test and record your parameters (including time of day).
Wait 7 days and do another water change (mixing exactly the same) and putting your same dose of weekly addatives in. Clean and reg maintainance.
Now test and record parameters. Repeat for 4 consecutive weeks and you should develop a consumption idea.
My 210 g system uses 30 - 40 ppm Ca each week and the Alk drop runs 40 - 50 ppm CaCO3. Once you have your weekly consumption rate figured out you can drip in your top off water with the appropriate amount of Ca and Alk supplements.
