show empty cell if result is more than 20

one way
=IF(((B$2-B5)/B$2)*100>20,"",((B$2-B5)/B$2)*100)

Thank you very much. It worked perfectly.
Next issue.
I am plotting the data in a line chart. Data versus calendar dates. If there is no data because the date has not occurred, the chart now drops to zero for those dates. Is there a way to plot only data that does not equal zero or plot only dates that have occurred without having to change the chart input data every time it is updated?

Thanks for your time!!