Sweetride, it is easily done and in fact built into the RK programming (ver 2.x). As long as you have version 2.x of the firmware, you can program the PC4 socket that the pump is on for ATO by first assigning it to a controller switched device, and whatever switch your using (SWA or B). Then you can set the run time property right from the menu. I find it is easiest for people to use the MyReef software to program this.
Another thing I do is I use 2 float switched, one about an inch above the other, and wire it all together as a collapsable circuit. Black wire to one float switch, red to the other, then the remaining float switch wires to each other. that way if the bottom float switch (the one that is set to the desired water level in the sump) fails, the other float switch trips and shuts down the ATO pump. I don;t know on your setup, but for some smaller tanks 2 minutes would still cause an overflow situation, so this other safeguard removes the failure point on the float switch.
If you need more help just let me know!
Regards,
Andy
(TeamDA)