I'm building a simple open shelf to place my radio on in the garage.
The shelf is 19" wide x 9 deep. I'm new at this so It will be a
practice piece to work on some decoractive touches.

I am going to put a back on the shelf that is flush against the wall
and extends down below the shelf. To this I will attach a few
turned dowels to hang stuff on. Two shelf supports will be of the
curved variety and brace the bottom of the shelf against the back
support.

Given the size of the shelf itself, are there ratios that are used
to come up with the size of the back piece that extends below
the shelf? I'm thinking about two thirds the depth of the shelf.

I picked two thirds because it seems to be a ratio that is used
alot - at least in the stuff that I have read.

Thanks

Ken