The 20% rule no longer applies to text on Facebook ad images as of 22nd March 2016.
You may have not liked it, but at least it was easy to define. The 20% text rule for image ads on Facebook has been in force for some time. They even provided a basic grid tool that allowed you to check how much of your image was obscured by text. Without any fanfare, and certainly no notification to their advertisers, they quietly dropped the rule.