Scale by constant amount shader

Hi, what I’m trying to make is a shader that scales the mesh down of an X amount, with X not depending from the mesh size.
What I achieved so far is some scaling, dependant on the mesh size, by multiplying the distance form the object center to the vertex position, the issue is that if I have a big mesh and a small one, the amount the mesh gets scaled is different. I also tried to subtract a costant along the direction of the normalzed distance I mentioned before, but this causes a “curved effect” as vertices closer to the origin should be moved less than the ones further away.

Any idea on how to solve this? It seemed pretty easy at first but it got tricky :3 Thanks!