ive never grown hydro myself, but this is how i think it goes
usually people check the ppm and ph levels everyday. if theyre off, they add a little nutrients until its at the right ppm, then adjust the ph if its off. when u add water, that lowers your ppms and can change ph a bit. depending on the size of the plant, u have lower or higher ppms. like for clones u want to start with like 0, then slowly go up. i believe when flowering they usually put it at 1000 or 1200.
putting nutrients in a bucket of water, then adding that bucket to your rez is basically guessing the amount of nutes. i recommend leveling off the water level with plain water, then check ppm and ph as stated above. start off on the weak side with nutes. its easy to burn the plants.
do not take this as fact. this is just what i learned from looking through hydro threads.