Josh, I have a few questions for you to help with the design. You really don't need a lot of design work, when you lay out your yard throat (the switches leading to the ladder tracks) the space taken up by the switches will determine how many tracks you can have in your space, and how long each track can be.
#1 The NMRA standard for track separation is 2" centerlines on adjacent tracks, is-2" from center to center of the tracks. The distance centerline to centerline is increased to either 2 1/4" or 2 1/5" on curves, I think it is 2 1/4".
There is one potential problem with that spacing. If you have trains on all of your staging yard tracks, the space between trains will be a tight fit for hands. If you need to re-rail a train, you may find you are knocking the train off the tracks that is next to the one you are re-railing. It also means that you need to set the first track centerline 2 inches back from the front of the staging yard, but again, you may not have room to get your fingers between the cars and the barrier or fence you erect to keep trains from falling off the staging yard.
#2 Are you going to have trains coming off the staging yard in both directions? Prototype railroads run either East-West, or North-South. I'm most familiar with the Santa Fe, so I will use them as an example. Santa Fe's mainline ran from Chicago to Los Angeles. Even though there were places where the trains would be running North or South, Santa Fe trains were always going either East or West by timetable. If you are going to have trains running in both directions on your layout, how will you turn them from the staging yard? One method would be to back the trains up from staging if you want a train to run in the opposite direction, but backing a train up a grade around curves may cause derailments. For those trains that need to back down grade to go into the staging yard, that is not so much of a problem. Gravity will take a lot of side load off the cars backing down grade. One trick you could use is to make a passing siding long enough to hold your longest train where the track to hidden staging comes to the mainline. You would then pull your train up from staging, and use the passing siding for the locomotives to make a run around move to go off in the correct direction.
#3. If you make run around moves at the mainline before going to the track down to staging so that all trains back down to staging, you will not need engine escape tracks in the staging yard. If you don't, you will need to have some engine escape tracks at the end of your staging yard to allow the engines to escape after pulling the train down to staging.
#4. I would suggest taking a couple of pieces of flex track and stick them temporarily in parallel to one of your shelves with double sided tape. Set the spacing at 2" and try to re-rail cars with another train on the adjacent track to see if it allows enough room to work.
I think going through all of these steps will result in your staging yard designing itself in that you lay out your switches for you ladder at the track spacing that works for you, and the space taken by the ladder subtracted from the total length of your staging yard will determine how long each track will be and how many tracks you will have room for in staging. If your space for the staging yard is determined by the amount of space you will have on the wall after you get down your 12 inches, it doesn't make any difference how much staging you want. The space you have available will determine the size of the staging yard.