Making borders a certain percent of a photo?

[Mundu]

I just realised that adding the same size border to every photo I've taken or cropped means some borders are going to look bigger on cropped photos and was wondering if there was anyway to make a borer a certain percent of the photo in photoshop?

 

[Mundu]

You have to remember though that the border won't be the same on all sides The longer side will have a smaller border than the shorter side.

Just realised that so that idea won't work. All I want is a border on all my photos that is the same size in comparision to the photo.