Regardless of which subroadbed approach you choose (plywood or spline) I would think a full size print of your track design would be desirable. You may find the plan doesn't quite work when scaled up, especially where switches are involved, but best to find out before you build. If you are not able to print one out, some kind of preliminary track laying, say on Homasote sheets, will vet the design before you start construction. Plans are notorious for needing to be changed.
Even where I designed the track work while fitting it, I was overoptimistic, and will shortly be rebuilding two derailment-prone problem areas
