Hi Aron,
How do you identify whether an apt is vacant or available? If it is identified by a flag in the database, the formula will look like this:
1) create a formula called @Vac_per with this code and place on the Details :
Numbervar vacant;
If {flag} = 'vacant' then 1
2) create a formula to display the percentage and place this on the report footer:
(Sum(@Vac_per)/count(@Vac_per)) * 100;
-Abhilash