I would not hook up the ro/di directly for top off - there was something on RC about the constant cycling of the ro/di that "passed" unfiltered water thru and on another thread, Dallas/Fortworth club one of the gals had it hooked up as you wanted, the water company was cleaning out the pipes with chlorine - lost the tank. Here some reading on the ro/di cycling - lots of reading but informative
http://reefcentral.com/forums/showth...hreadid=553571 He circumvented this by having the ro/di unit fill a "holdoing" tank then from this fill the sump as topoff.
I'm with R1 on those "bottom" holes - just use for a CL and you won't have to worry about flooding or draining the tank.
hth