Hudson,
This is another consideration where you will get a variety of answers. I think you need to take into consideration if only adults will be viewing and operating the layout or if children will be viewing it too. I take it from your comment that 40" seems too low, that you either aren't planing it for small children, or children will be seated on stools. My personal opinion is that 40 inches is too low for me. I would be straining my back bending a bit all the time I am reaching onto the layout. My last layout was 50 inches high, which gave a nice realistic viewing angle, but I am going with a little lower than 45 inches on this layout because I have a big window between my train room and the adjacent room which has a 45-inch high sill. I want to be able to look into the room and not see the side of benchwork. The trains will climb 5 inches to the benchwork on the other side of the room.
I don't like it too low either, because I have arthritis, which affects my hands, but now is attacking my knees. I don't want to crawl under a low layout. Others have said they want it at a height where they can lay on a roll around creeper and have the layout at a comfortable height for reaching up to do wiring.
You will get a host of different opinions, which is great. You will then be able to pick through them to see what best suits your needs.
