From my understanding, the level is for filling when the system is cold. I've always operated that way.
Now let me address the "disappering" coolant problem.
I had this happen to me for weeks and weeks until I took the advice of people on this post.
When you install a new radiator and hoses, the main sources of leaks will be those you can't see (!) when the system is hot and coolant is escaping as vapor.
Why? Because you have new hoses, clamps, and parts (radiator) that are undergoing the first stages of thermal cycling and ..... you guessed it .... you have to go back to each connection and ensure it is leakproof by tightening the clamp, etc.
AS an analogy, I had to do this three times on the Y-pipe cat-back ******. I couldn't believe it needed additional tightening as I had tightened it very hard each time only to find I could tighten it some more after successive thermal cyclings.
One cause of leaks - I "thought" I had tightened the clamps enough on the hoses to the radiator and didn't want to overtighten to crack the plastic.
Well ... guess what. When I used a radiator pressure tester to pressurize the system cold, the leaks were occuring there and stopped when I tightened the clamps further.
And that's the last option if tightening the clamps doesn't work. Rent a coolant system pressure tester from Auto-Zone for about $80 (fully reimburseable when you return it - except for the tax!) and pressurize the system COLD to the level on the instructions.
You will see coolant squirting out everywhere (!) maybe like my system.
After finding the leaks (or perhaps having to replace a heater hose), the system will be buttoned up tighter than a "frog's ass in a pond" as the hicks say.
Absolutely no leaks from my system after that.
The reason for the pressure tester is to test the system COLD as with a hot system it is almost impossible to find the leak without burning yourself and the leaking fluid may be invisible as it vaorizes out of the opening.
Be forewarened, that once I replaced the radiator, in a short period of several weeks I started to loose other hoses - heater, crossover hose for engine, oil cooler - I guess they all start talking to one another and decide to drive you crazy.
But for me, even after replacing the faulty hoses, I started to get ****** off because I couldn't figure out what hoses could be left - I replaced them all (!) - and so I rented the pressure tester.
Good luck and don't give up. You will get a completely leak proof system and everytime you look in the coolant bottle you'll be ****** off because you can't ADD coolant!!!
