First, you do need to calibrate your dosers to see how much they're putting out..... they can vary by a tenth or two per minute. Not a huge difference, but it can add up. My BRS dosers do 1.2ml/min.
Next thing you need to calibrate is your tank.... how much 2-part to raise a given amount of Calc & dKH. Test your Calc & Alk, then add a measured amount & let it mix it in your tank for about 15 minutes & test again to see how much it came up. For instance, in my 28 nano, I know that 10ml raises my alk 0.5dKH (I make my own 2-part, so don't go by that number.... test yourself).
Now find out your usage over a 24 hour period..... get up to where you want, then test 24 hours later. I know that if I lose 0.4dKH over a day's time, I need to increase my daily dosage by 8ml (6-7 minutes).
Lastly..... pick a range that you're happy with. I keep my tank in the 10's (between 10-11). If you try to keep it right at 8.2, you will drive yourself absolutely nuts trying to maintain a specific point.
Even though I'm pretty well dialed in, I still test at least twice weekly. I have some very fast growing SPS (digis & birdsnests), so each tip that splits is consuming twice as much nutrients...... when you've got multiple corals with lots of growing/splitting tips, consumption can increase fairly quickly. I find that about every 3-4 weeks, I'm increasing my daily dose by about 5-10ml.
