Facebook has a 20% text to image ratio for its ads, however this ratio is worked out on a 5 by 5 grid.
This tool allows you to quickly check if your image is compliant.
You should read this post for more information.
Simply upload your image. Once image has loaded, select the boxes with text, and the tool will work out its percentage and compliancy. That's All!
Thanks for trying it out, and remember to share if you liked the free tool!