I had a slightly different points system which I used when I did this previously, and it worked out absolutely perfectly. But unfortunately I've lost it. In any case, it was a bit complicated, and I wanted to make this system really easy for people to understand. So everyone could work out their own scores if they wanted to.

Also, I want to reward people for tipping the winner on top as well. Sure it's great to get a box trifecta or a box first 4, but those people shouldn't get a substantially bigger score than someone who manages to pick the winner on top.