with the n-r point scheme, realize that 1st place will get 19 points:
1st, 19.00
2nd, 18.00
3rd, 17.00
4th, 16.00
5th, 15.00
6th, 14.00
7th, 13.00
8th, 12.00
9th, 11.00
10th, 10.00
11th, 9.00
12th, 8.00
13th, 7.00
14th, 6.00
15th, 5.00
16th, 4.00
17th, 3.00
18th, 2.00
19th, 1.00
20th, 0.00
Other option is n-r+1:
1st, 20.00
2nd, 19.00
3rd, 18.00
4th, 17.00
5th, 16.00
6th, 15.00
7th, 14.00
8th, 13.00
9th, 12.00
10th, 11.00
11th, 10.00
12th, 9.00
13th, 8.00
14th, 7.00
15th, 6.00
16th, 5.00
17th, 4.00
18th, 3.00
19th, 2.00
20th, 1.00
I don't quite understand your rebuy thought. It is more common to penalize people points wise for taking a rebuy rather than rewarding them with points (as Phaze is suggesting).
Then your formula would be n-r-nr (or n-r+1-nr) or as phase suggests you could use n-r (or n-r+1) and include the -1 points in the "points for rebuy" section of the game tab.
So, mention what points you want for all positions (1st - 20th) and also decide on what you want to do if only 15 guys show up (do you still want 10th place to get 10 points? Then the equation changes).