proportional beveling script?

Hi, does anyone know of a proportional beveling script?
What i mean, is that the beveled vertices don’t travel a fixed distance along their edges. They should move a certain proportion of the length of the edges they are on.
This way it is impossible to break meshes by beveling too much.

So, does anyone know of such a script? I already tried to modify the bevel script that is shipped along with blender (2.36), but didn’t succeed… :frowning:
Maybe someone else will manage.