IMO 16.5ml per day is not much. In your 30 gallon system (not including sump and rock displacement) that only equates to .75dKH consumption per day. Calcium and alkalinity consumption are balanced at 2.8dKH to 20ppm CA. So in your scenario your system is only consuming 5.355 CA (if your system was perfectly balanced and 16.5ml was your end point for alk). Problem is, virtually no ones system is perfectly balanced. The biggest culprit of causing the imbalance is CO2, this is especially evident during the winter since we close all our windows and virtually no fresh air is allowed into the system. Typically alkalinity consumption will exceed CA consumption for most aquaria. Just to give you an example, my system consumes 160-170ml per day CA and 210ml alkalinity per day.
a 5.355 consumption of CA will be hard to see on our hobbyist test kits unless a week or 2 goes by between testing. At this point I would stop dosing CA until you can get the number below 480, 450 or below would be ideal though.
Here is what I would do if I were in your shoes. Turn off the CA doser until the CA was around 450. Keep the Alk doser on at 16.5ml per day and manually add buffer at a rate of .5dKH per day (use the calculator linked below to get this amount) until my alk was at 8.5. Once the dKH is 8.5 write down the time and turn off all dosing pumps. Test the alkalinity and write down the figure. Exactly 24 hours later test again and that will be your rate of consumption. Use the dosing calculator and set your dosing pump accordingly. Then test every 2-3 days and adjust accordingly. Chances are it will shift up or down a bit and if you test daily to adjust your pump the noise from the test kit could have you chasing numbers for a while.
Hope this all makes sense, let me know if you have any questions.