I have done a bunch of these but for some reasons I keep getting really weird and different amounts when I try to calculate base material 5x7ft landing 30" tall going to have blocked sides. 4ft wide 18ft long ramp need 8" of base at the lower end and 30" at landing. Trying to help out a family with a handicapped child. They started by over excavating the area. How much compacted base will I need?