This should do the trick for you:
if (r<=10, (n*3)-(3*(r-1)), (n*3)-9-(2*(r-1)))
Here's the points breakdown, for a 23-person tournament:
1st, 69.00
2nd, 66.00
3rd, 63.00
4th, 60.00
5th, 57.00
6th, 54.00
7th, 51.00
8th, 48.00
9th, 45.00
10th, 42.00
11th, 40.00
12th, 38.00
13th, 36.00
14th, 34.00
15th, 32.00
16th, 30.00
17th, 28.00
18th, 26.00
19th, 24.00
20th, 22.00
21st, 20.00
22nd, 18.00
23rd, 16.00
Good luck!
Orrie