Try this:
IIF(Sum([NACTTIME])=0,0,Sum([NSQFT])/Sum([NACTTIME]))
The IIF function has 3 parts (separated by 2 commas). It basically says, "When this condition is true[1st part], in this case "Sum([NACTTIME])=0", do whatever it says in the [2nd part]. (That's when you might want...