Rev, the problem with your setup for auto topoffs is I amoung most of us do not have enough room where our display tank is to have a trashcan to do that.
my problem is I dont know how much water to add for a top off - if i add to much then i need to add some salt/ if i dont add enough my SA is too low.
I mark my sump so i know to add up to that line....it seems to be the best and only way to know
i do need a RO
unit but in the mean time I have done my topofs with distilled water