Arcs for bending on edge

Let's see if I can explain this --

I'm doing a drawing for an aluminum chassis and I want a 90 degree bend exactly on the edge of the material. (062 aluminum)

Now, the way it's supposed to be done is that you cut/punch a couple of arcs on each side of the bend, right?

Here's my question:

What are the guidelines for sizing these arcs? For example, should the arc radius be 3-4 times the size of the material thickness? (more? less?)

Say, the "O" is actually an arc (half circle) and the bend is between the 2 "O's"

--------------O----------------------O---------------