IF(Paid>0,({Profit}/{Paid}*100),"")

This is my current formula after playing with it for some time. I am calculating ROI, with many of my records containing no initial investment, which left the result of {profit/paid} as infinity on those lines. I want to easily calculate my average ROI, so I can’t have the infinity records. Initially, I thought I could add to the formula IF({profit/paid}=“infinity”,""), but that formula failed, so I ended up with the above. The problem is, because of that formatting, I now cannot format the formula to show results as a %. Any ideas?