Distance from wall - Model Railroader Magazine

This is where you have to figure out what your criteria are for all the major characteristics of a 'successful' track plan are.  If you want maximum scenicked area, go wide, but keep the trackage closer to the front...where your fascia and controls are likely to be.  If, like Rob, you can model narrower shelves and do such a fine job of it, you can still have slightly sinuous trackage, or tangential trackage, and still have enough filler scenery to make images successful and your contemporary enjoyment successful.

On the other hand, if, like me, you like to run long cars on passenger trains, you'll want larger curves.  As a general rule, the wider curves need deeper benches, or they'll need angled corners nearest the fascia.

Your stated height also presents a bit of a problem unless you're very tall.  One's elbows tend to snag delicate things near the edges of the layout when we reach deeply into it...deeply because we've filled wider benches and need to fix or to place, or to retrieve, something.  Also, for those of us altitudinally challenged, high layouts restrict our ability to bend at the waist to extend our reach.  That awkwardness forces us to use stepping aids or to risk those deep awkward reaches that cause problems at least once a month.

I tend to go to about 30" for the sides of my long room, and deeper at the ends where my sweeping curves are.  Thirty inches is at the limit for me where my bench's main height is 46".  Then I have to drag a foot stool.

Lastly, this is where you can mock it up and try for yourself.  Also, do use graph paper and use a convenient scale to map it all out.  Use a compass to accurately depict all your curves where they will serve you best, including accurately scaling out the radii.  From there, you simply draw the front of the bench away from the walls where the tracks will allow train items to tumble off and not roll over and onto the floor.  Measure it out to 1:1 scale and you'll have your bench depth there.  If it exceeds 30" above about 45" in height, consider some alternatives.