Josh, there are a couple of ways to set your grade for the under layout staging yard access. The easiest, but somewhat expensive is with a gadget sold at Home Depot. At our railroad club meeting a couple of months ago, one of the members showed us a tool he bought at Home Depot. It is a digital grade indicator. It sells for @ $40.00, but you lay it on a board and it tells you the grade. I think it is made for contractors who desire to put a little slope into a patio for drainage, check roof pitch, or any other job where they need to have something pitched and want to build it to a specific grade.
The Feb. 2008 issue of Railroad Model Craftsman has an article on how to make a "grade setting gadget" by David Wiggs. He uses an 18 inch carpenter's level, and two pieces of lexan 4 inches long. He includes a chart that calculates grades based on the size of a standard fractional drill bit used at one end. It is a little more complicated to use than the digital system, but the materials to make this gadget is a lot less expensive than the digital grade setting tool.
The third method requires a measuring tape or ruler and doing a little math. You find the percent of a grade by dividing the drop by the distance. First everything has to be converted into thew same measurement. As an example, lets presume you are going to make a drop of 3 inches along a 10 foot wall. First convert 10 feet into inches-10 feet x 12=120 inches. Now, if we divide 3 inches by 120 inches, we get an answer of .025 which is a 2.5% grade. You can go around your room and starting at your mainline grade level, measure down the distance you estimate you will need to get to your staging yard level, and make a small pencil mark. Then measure the distance along the wall from where you started. Divide the distance in inches into the drop in inches. If the grade is not steep enough, increase it, if it is too steep, decrease it. If you get 4 cheap wooden yard sticks, cut one into 1 foot lengths. Now carefully lay out the three full length yard sticks, and splice them together with the 1 foot sections of the yard stick you cut up. If done carefully, you now have a 9 foot long straight edge. If you plot 2 points on your grade that are less than 9 feet apart, you can use the straight edge to draw a line on the wall through the two points to get a 9 foot long grade. Use the line to plot where to install the brackets to support your grade down to staging. You do not need to use the high dollar brackets that you are using for your main layout or for the staging yard. Presuming a double track going up and down to staging, your sub road bed should be about 4 inches wide or so. At Home Depot or Lowes, you can go to the lumber department and find a shelf full of various steel brackets designed for construction by Simpson Strong Tie. I have found some small 90 degree brackets that could be screwed into the wall and under the sub road bed that would be adequate to support a 4 inch shelf. When you get to a corner, lay out your curve on a piece of plywood, and cut it out. Now when you lay that curve into the corner, you know where the straight sections on each wall need to end to fit the corner section in. Cut 2 straight sections of plywood long enough to overlap both the end of the straight and into the curve and screw and glue it to the bottom of the end of each straight and the ends of the curve to support the ends of the curve. You may not be able to support the center of the curve from the wall, because it will be too far out from the wall; but you can put in your supports as far into the curve as they will reach. I picked up a couple of 90 degree brackets that were @ 1.5 inches on one leg and @6 inches on the other leg to use for reinforcement for a redwood swing set I built for my grandsons.
By the way, flooring companies often have "give away" wooden yardsticks with the company logo that they use for promotional purposes. Another long straight edge would be a framing level. I bought a 6 foot long aluminum framing level from Harbor Freight for @ $20.00.