first, I use the Lucas Formula with GH Nutes...I never change my res...I just add rod water and/or nutes to bring ppm to 900...ph is very very stable, I havent used up or down in 3 harvests( I regenerate/reveg )...I test the res daily anyway to monitor ppm..
second, I used to change out the nutes on schedule, I used a pump, the one they sell at walmart for kerosine...this made the changeout much easier...(with a 2 way valve, you could even pump it out from the res with the same pump used to move it from res to roots...)
third, I used to drain into an open topped res...now I use a covered res and direct the drainflow into pipe fitting into lid of res...no big difference, just less algae and evaporation...
I hope this covered your concerns...apparently, it's not rocket science
