I think you did fine the first time you did it. I don't think you overfilled. You could leave about an inch below the neck for expansion, but that's all you should need to do. I'd consider that you have a bad radiator cap, and it wouldn't hold the pressure. Needs to be a 16 lb cap.
The 2nd time, I think you just let it get too hot without the cap on. Don't run it until it gets hot without a cap. The cap is needed to maintain pressure on the system, which keeps the coolant from boiling when the temperature gets high. If you leave the cap off until the thermostat opens, that's 195 degrees or more that's going to blow out the radiator.
So, I suggest making sure you have a good cap, fill it up, put the cap on, and start it. Feel the upper radiator hose after a few minutes, to see if it gets hot. When the hose gets hot, that's how you can tell when the thermostat has opened. And that will verify that the t-stat is working.