Yes it does start and stop hence the FAIRLY constant comment but my TDS are low. RO/DI been up for quite awhile with the same filters and believe that's the reason for the 7ppm as up to a few months ago a lower TDS was more common.
As for the RC idea seems complicated to me just to add some water. Had at one time planned similiar that would just fill a large vessel every few days (by timer and float, or just manually opening a valve but with the float in the holding vessel in case I forgot to turn off) then gravity feed sump through the float valve thats in the sump. Until that was setup I put the ro/di direct to the float. Ended up it's working fine as is so jut left as is.
Getting back to the range I have (1/4" variance) perhaps thats the reason for my low TDS as opposed to waiting (time between cycles) to drop 2 inches.
picture of the valve I'm using in this thread
http://216.187.96.54/vbulletin/showt...ighlight=float