I’m not 100% sure either. But, you could make sure the radiator is full, get it to operating temperature (thermostat opened) and fill it to the HOT FILL mark....
Just make sure you have at least some in the bottom, and the hose is pushed all the way to the bottom... that way you don’t suck air in instead of coolant.
Also, until they went to the pressurized degas bottle, the coolant tank was more of a pollution control than anything else. In the 70’s and earlier, there was just a hose run down the side of the radiator and what ever came out just dripped on the ground. You will fill it up and once all the air was worked out, it would find it’s own “full” which normally isn’t up to the neck..... the coolant recovery bottle was just that.... to keep it from dripping out on the ground. It does have the side effect of allowing the system to draw coolant back in..... but I don’t think that was it’s real design purpose. I worked on a 56 Packard for a lady and it hadn’t ran in a bit. Someone had filled the radiator completely full after the last time it had run. once I got it running, the “extra” dripped out on the ground. My oldest (25) was worried something was wrong and we had a leak. He is used to the “newer” vehicles (his DD is an 86 IDI). I showed him the hose and explained how it worked....