Create your formula for the 1 and 0 in the Formula Expert - I'm going to call it MCData_1or0.
Create another formula to do the sum - I'll call it MCData_Sum. If you need the sum at a group level it will look like this:
If it's a total sum for the report it will look like this:
To do the percentage, create another formula that looks like this:
If {field or other sum formula} = 0 then
0
else
You need to make sure that you're not dividing by 0, hence the first part of the If. The '%' operator does the percentage calculation (a/b*100) for you.
-Dell