What were you using? The filter should be MERV14 or higher. It will remove >75% of all small particles too.

You shouldn't end up with 13% leftover, if you're doing it right. Air will pass through the filter multiple times, and it removes >75% each time.

Below MERV14, you'll have that problem. Above, it should do as well as HEPA. The gap between 13% and 0% will be made up for with a 13% stronger fan.