It is really close btw, but having heavy cardstock sure makes everything that much better in the long run.
Even though it is hard to see from this angle, I believe that where I put the red line would be where you could reinforce a wooden stick on the inside of the helmet to correct the slight variance.
Also, I was appreciative of the fact that you were using superglue. I see it is the gel type, and I have not experienced that before, but I am very partial to the brush on superglue myself.
Also, don't be afraid to cut a seam that went wrong in order to correct it. You can always use a backer piece of cardstock to rejoin the pieces while at the same time, making the correction.
Another tip I have is when gluing two pieces together, start in the middle of the two. Example: If there are 5 tabs on one, and 6 on the other, start on #3 with the one with the 5 tabs. This will help make sure that all the pieces align properly.
Lastly, as you pretty much stated, practice makes for perfection! Keep it up, you are doing great!