For Uc Nutes, Do I Add System Gallons + Rez Gallons To Get Xml/gal?

I'm trying to figure out how to mix my top-off Rez for week 2. System is 100 Gallons. I plan to only mix my next week nutes in 20 gallons in the top-off reservoir (i'm guessing that's the max they'd drink at week 2). So am I calculating the nute schedule based on 100+20=120 gallons? So for instance, they recommend 1ml/gal of Veg A and Veg B so I put 120ml of A and 120ml of B in my 20 gallon top-off rez?
CC recommends running your top off at the same pH and ppm as your system. So if you're running a 20 gallon top off, just use 1/5th of what you used for your 100 gallon system and you should be golden. If you add 120 ml of both A & B to your top off you are likely to see your system ppm go sky high in a day or two.

I've never reached the levels of nutes that CC recommends in their feeding charts. In these UC systems I find that about half the recommended amount is a good starting point.