What most people do is locate the counterweight setup where is convenient & allows you to open the door (some position it to be a stopper as well) & then you just keep adding offcuts of round bar, flat bar or whatever you have lying around, in the top piece of the counterweight till it feels right & allows easy opening of the door. Then you just weld it all inside the top piece to keep the pieces from rattling & moving around, then just weld some end caps on & bobs your uncle, you have a neat, perfectly weighted counterweight!
To add my 2 cents (and I don't have a brother, uncle or a prime minister...lol) I attached my up bars about 1/3 of the way down the door and 30 degrees or so. With my door fully open, the weight lays nicely on the cooking chamber making a simple but effective door stop.
I believe that the weight should help open AND help close the door. My door is perfectly balanced at the half open position. I can let go of it and it will just sit there. You can balance it how you want.