Try the following:

**max(17-(r), 0)**

This should provide the higher of the two variables, either 17 minus rank, or 0.

For 1st place, 17-1= 16 points

for 2nd place, 17-2= 15 points

for 3rd place, 17-3= 14 points

...

...

for 16th place, 17-16= 1 point

for 17th place, 17-17= 0 points

for 18th place, 17-18= -1, in which the equation will award 0 points instead because 0 is greater than -1.

...

...

for 48th place, 17-48= -31, in which the equation will award 0 points instead because 0 is greater than -31.

This should work for what you need.