even if you are really off a fair amount, targeting 10-30ppm is easy to day.
there's not much need for anything over 30ppm I am aware off, so that's more a max amount you'd ever want, if you have less light etc, then you should consider adding less.
I've not seen many tanks that use more than say 25-30ppm per week of NO3 as well, but you want to maintain and residual for optimal growth, not go from 30ppm down to zero or close to it by week's end.
So keeping a mid range of 20ppm is not bad either. If you have less light, sediment based ferts, then you can get away with more variation/less since you have a back up supply and with less light= less CO2 demand= less nutrient demand= less change= less growth.