Really fun concept - and you're right, I *am* bad at math!
A couple of suggestions for improvements:
1) Could the number of the next tile be displayed where the arrow is? I found I was looking at the right waiting for the perfect number, only to discover I had lined the arrow up in the wrong place!
2) I'm not sure the distribution of numbers is very helpful. My target was 9, and I mostly got 6 and 7 tiles, which made the game very difficult.