I got big headache with my chiller too initially. But once u got the flow rate rite for your chiller, the rest/ kick time will be pretty consistent.
Another thing to add which I'm not sure if it apply to all resun chiller is the temp probe inside the chiller. There's a diff in the temp in the chiller as compare to either the sump or main tank. Therefore, to get a more accurate temp, it's better to control the chiller using an external probe!
Happy reefing!!!