Proper coolant fill procedure?

Start with a gallon of full strength coolant. This guide can help you.

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

If you overfill it, the coolant will overflow and run on the ground when the engine gets hot. Other than making a mess, that usually doesn't hurt anything, but if it makes the serpentine belt wet, that may squeal for a while until it dries off.

You have me confused too. Most people don't ask how to dump coolant into the reservoir, so I have to assume you're running into some kind of problem, but you didn't say what that problem is. Don't get angry with me if I can't read your mind. If you have a specific question, ask it.

Yes, you have to pour the coolant into the reservoir. The engineers at GM couldn't be bothered to put radiator caps on most of their radiators. To make the coolant go into the engine, you have to let the air out. There should be a bleeder screw for that on top of the thermostat housing.

I can think of over a dozen legitimate questions you might have, but you're going to have to say what those are if you do have any. Yes, I did read your post thoroughly in an attempt to figure out what problem you're running into. If you reread it, you'll see there's no clue indicating what kind of help you need.

I'm missing something here. It sounds like you know more than the average car owner about this. It's hard to break it down into individual steps, so let me tell you how I did this at the dealership, then you tell me where my thought train went off the track.

After flushing the system with water from a garden hose, there will always be some water left in the engine block that doesn't drain out. The best freeze protection you can get is with about 55 percent antifreeze and 45 percent water. A 50/50 mixture is standard. Some people like to premix the water and antifreeze so they know it's exactly half and half, but that doesn't take into account that water that didn't drain out. The way I handled that was to add one gallon of straight antifreeze, then one gallon of water, which would usually just fill the system through the radiator. The reservoir got flushed with the rubber hose removed from the radiator end, and by letting it hang down, it would siphon the water out so it was empty.

Some engines didn't need to have the air bled out. Those that did either had a bleeder screw to open or there was always a temperature sensor near the thermostat housing that could be unscrewed. Once the coolant was filled high enough to run out of that opening, it will hit the thermostat and cause it to open when it gets hot. Close the bleeder screw or install the sensor. At that point any air left in the system will work its way out on its own.

At this point I'd run the engine to warm it up and make sure the thermostat opened. The water and antifreeze would mix by that time. I usually left the radiator cap off and just covered the opening with a piece of cardboard from an oil filter box, and put a small weight on it. That prevented pressure from building up and the expanding coolant would still run freely into the reservoir.

I tested the freeze point after running the engine ten or fifteen minutes, then I added water or antifreeze as necessary to the reservoir to adjust that temperature. After one or two warmup cycles, whatever I put in the reservoir would get mixed in.

There's two tools for measuring the freeze point but they don't work the same way. The most common tool is the plastic or glass cylinder you suck coolant into, then a pointer or various discs or balls float to indicate the freeze point. With these you have to understand that they are measuring the weight of the coolant compared to the weight of water. That is only accurate down to about minus 50 degrees F. From that point on, if you add more antifreeze, more discs will float making it appear the freeze point is lower, but in reality, once you get past about 60 percent antifreeze, the freeze point actually starts to go back up. Water freezes at 32 degrees F. Antifreeze, if I remember correctly, freezes at around minus 10 degrees F. It's when you mix them that the freeze point goes lower. Since antifreeze is much heavier than water, a higher concentration makes the indicator pointer or discs float higher even though the freeze point is not any lower.

The other tool is called a "refractometer". Don't ask me how they work, but you place a drop of coolant on the glass window, then sight through the tool. Part of the viewing window will be dark. The dividing line between the light and dark areas is next to a scale where you read the freeze point. Those only work if the glass is perfectly clean of dirt and other contaminants.

Since most of my customers were repeat customers, I always rechecked the freeze point at their next visit, and added a little water or antifreeze to the reservoir if it was needed.