I would shoot for around 5 days worth of evoration time before the pumps ran dry. That would require between 5-10 gal
of water in the pump area depending on evaporation. I see about 1 gal
a day on both of my 80 and 75 tanks. This would allow you to go on a multiple day trip an dnot have to worry about having someone come to the tnak every day to dump some water in. When you do have to have someone assist you with water additions you should add a line with marker or water resistant tape to the outside of the sump to mark the FULL level and also mark a line just above where your pumps start to suck in air and mark it LOW or ADD WATER.
A way to automate this would be to have a float switch setup with a RO
filter that would start the RO
filter when water dropped and caused the float swtich to open. Then as the RO
filter filled the area it would close the switch and cut the flow of water. An automated procedure would permit a smaller pump chamber. When calculating sizes remember your going to be running the tank at 50-75% full during normal operations so you will only have between 8-12" of water in the system vs the full 16" deep if it was full.