This is so creative, this mechanic has sooooo much potential. Like others have said, I also have had difficulty casting spells. I think to help fix this problem, you should take the right approach. You have said that many people make "mistakes" like having the circle more close to the middle, and that makes the spell not cast. I think instead of enforcing the "correct" way to cast a spell, you make the detection way more lenient. The wow factor of the mechanic shouldn't be "wow I finally drew the spell perfect so now it cast", it should be "wow the game knew what my scribble was and I actually casted a spell just by drawing, I feel like a wizard". Instead of correctly people how to draw correctly, embrace the mistakes and let everyone be a wizard. Plus, not everyone naturally starts their stars on the lower right corner. I still enjoyed playing this a lot though, when things work it is very fun!