I run a rdwc *with* a top-off res. I use it to stage all of my res changes, and keep the RDWC at the same water level at all times. You can also use it to steer the EC/PH.
1. Flip a valve to fill res with RO water, add nutes and PH.
2. Valve off the res, drain the RDWC.
3. Valve on, fill up...