I just started dosing the ferts last week, so I am pretty new at this as well. The way I understand it is that you want to maintain the Nitrate level at 5ppm. That correspond to the ideal level, or you target level. Therefore if you have 0 Nitrates, you would need to dose enough mL of the solution to bring your Nitrates up to 5ppm. How often you do it depend on how fast your plants use it. For the experienced guys here their plants are so well developed that they need to dose it every other day. In my case I dosed every other day, and checked my Nitrates levels they were through the roof. That tells me that either my plants are not using it as fast, or I have few dead fish decomposing somewhere in there. At the curren time I only need to dose twice a week to keep my Nitrates at the target level. I think it is almost imposibble to keep the levels at exacly 5ppm. For examle in my tank I am aiming at 10ppm. What I do currently is dose enough nitrates to bring it up to 15ppm, as the days pass by these get used up. Since I don't have the tank figured out yet, I test everyday for nitrates, once these drop to 5ppm, I dose again to bring these up to 15ppm. It could be 2 days, sometimes 3 days.
Sorry for the long reply, I hope it helped. If this is not making sense please let me know because maybe I am doing something wrong as well.