specify percentage for "free disk space"-limit

This might be a feature request or maybe I'm not looking hard enough: In the settings for backintime, you can specify something like, "delete backups when free disk space is less than ____ GB". I think it would be amazing if I could say, "... is less than 30% ". Of course this would require that backintime does on the fly math before making its decision, but I'm sure its very do-able. If this is something easy to do myself, please tell me "you do it!", I'd _love_ to write it myself (I studied python for a little while, but haven't touched it since).