How much coolant needs to be added to radiator?

I drive a 2001 Buick Century. It has had some coolant problems/leaks on and off. My father in law was helping me with the car, and he told me that the radiator must be filled to the tippy top up to the cap (in addition to checking the overflow tank at the hot/cold levels appropriately). I’ve been unscrewing the radiator cap in the morning before work to check it, and it will consistently take about 4 oz of coolant to become filled to the top top of the radiator, to be filled all the way up the cap. This makes me wonder if I’m losing fluid, since I’ve been doing this everyday for a week or two now, but I also see no evidence of a leak like I used to…

So, my question is this:

I’ve been doing some research, and I’ve seen a few people say that the radiator cap only needs to be filled to the bottom of the neck, but the neck doesn’t need to be filled. I’ve been filling the neck too, up to the top of the cap… Is it true that I only need to fill up to the bottom of the radiator’s neck? Because the sources online saying ‘fill up to the bottom of the radiator tank’s neck’ are making me suspicious of what my father in law has told me to do…Am I damaging the car if I’m filling the radiator to the ‘tippy top’ as I said? Or does that excess simply get drained into the overflow? Or just get spouted out somewhere?

I really don’t know anything about cars, I used to live in the NYC area, please be easy on me.