An ATU should only top up a little but of water each time it runs. Percentage wise I never though about how much exactly. I got the SEGA unit and the two sensors are placed relatively close to each other. From the time it hits the lowest float switch. It must trigger the top float switch within a minute, else an alarm will sound. Mine is set to a minute. The SEGA can do 1, 2 or 3 minutes, or no timer option.
Due to it being a relay switch, you can use any 220V pump on it, regardless of size.
You do not want your ATU to fill 5% or more of your system volume every time it runs. Reason for ATU is to keep the salinity as stable as possible.