Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
Be aware that as the liquid level in your reservoir goes down, it weights less and exerts less pressure across the flow restriction; which means less flow.
So the dosage when the reservoir is nearly empty will be quite a bit less than when the reservoir is full.
You can see this for yourself by hanging a Ziploc bag full of water from a top corner, then poking a large enough needle hole in the opposite corner that you get about 1 drop per second at first. Count the drop rate when the bag is 1/2 full, 1/4 full, etc.
You could compensate by using a larger reservoir and topping it off often enough that the liquid level doesn't change too much.
But if you're able to build a controller, you could also drive a pump (which can also be DIY'ed and cheap) that doesn't rely on gravity.