I really don't know much about it, but I think the place that gets half of the amount that 1st gets should be at least 3rd, not 2nd. I feel like it would be too easy to become uncatchable. Maybe it goes like this, where the points are the number of entrants:
1st - 100%
2nd - 75%
3rd - 50%
4th - 25%
5th - 1 point if 4th doesn't get 1.
Just as well, should we try to organize a prize pool of some sort?