Question on adding coolant to radiator

The car is cold and hasn't been on for a week. I pulled the cap off and I saw that the coolant wasn't over the metal cooling passages. I added almost a full jug of coolant and it still didn't even submerge those passages inside.

I checked the cap a few months ago and there was some coolant inside the radiator (car was warm). I know that a little overflowed out the other day from the overflow tube but didn't think it could use that much up.

Should I keep filling it until the coolant is just below the bottom part of the cap?
Just wanted to make sure that I do not over fill it because it's cold and I haven't started it in a bit.

It has a 318