Crystal actually has a percentage operator - '\' (instead of '/'). Also, you need to be careful to make sure that the number you're dividing by isn't 0.
I would create one formula for the DistinctCount, like this:
DistinctCount({GPC_GRADE_DISTRIBUTION.STU_PIDM},
{@instr})
Then do this in the formula you show above:
else 0
When I place a percentage on the report, I then format the object to include the percent sign like this:
1. Right-click on the formula in the report and select "Format..."
2. Go to the Number tab and select "Custom Format".
3. Go to the Currency tab (I think that's what it's called... ) and turn on the currency sign. Change the position in the drop down so that it comes after the number. Change the currency sign to the percent sign.
4. Click OK and OK. You're number is now formatted to include the percent sign.
-Dell