Well BG i'm sort of in the same boat as you in that im trying it for the first time :) I have 50g top off res's that I nute up. Basically i've just added about 2ml/g more than what I put in my systems. For example 3rd wk of flower I was at approx 5ml/g A+B in my UC, so I added 7ml/g A+B to my reservoirs (350ml).
So far it seems like my pH and nute strength is fairly stable...it's dropping slowly, about 20-40ppm/day in the UC (with the top off nutes filling the UC). Whereas before I started nuting my top off they were eating around 80-100ppm/day in the UC. So anyways..I dont have the experience to tell you one way or another how to do it since im not sure, but you can take what I did fwiw. Good luck.