As you can see from the photos, I have now laid down all of the track supports. It all looks a bit nasty and untidy but it is all that a small N-Scale layout needs and it will all be covered up anyway. The plan to start everything from the middle seems to have worked. I laid down the wooden board along the front of the board and that established a level pretty much in-between the baseboard and the highest I needed to give track clearance over another track. This has had the effect of reducing the gradients required down to around 2 - 2 1/2% which is really good on a layout this size. I have had steeper grades before and they always give a bit of trouble. Next step, over the weekend, is to start laying some track.