trigs;399039 wroteeasiest point system (for 20 members) would be to have points based on number of people. i.e. 20 - 1st, 19 - 2nd, 18 - 3rd etc.
Other leagues I know of use a formula similar to this one:
=ROUND(SQRT(# of Players)/SQRT(Position of Elimination)*2,2)
20 ppl, 1st = 8.94, middle = 2.83, last = 2
15 ppl, 1st = 7.75, middle = 2.93 (7th) 2.74 (8th), last = 2
10 ppl, 1st = 6.32, middle = 2.83, last = 2
5 ppl, 1st = 4.47, middle = 2.58 (3rd), last = 2
You can adjust the points given by changing the multiplication factor if you don't like all the smaller numbers
In anticipating a field of 20, by changing the multiplication factor to 22.36,
1st place = 100.00 pts, last is 22.36
15 runners: 1st = 86.6
10 runners: 1st = 70.71
5 runners: 1st = 50
etc...