John, My current layout is 43" high with the highest track level at 50". I am 5' 11", and it works out well for me. My last layout, which was a long time ago, used L-girder. Places that didn't have roadbed and track were covered by extruded styrofoam which was the base for all my scenery. It worked out well. I didn't have any heavy buildings, but if I did, I would have built plywood bases to support their weight.
I used the method of setting up a temporary table with cardboard boxes to represent different heights before building that last layout which was a little higher. This time, I just winged it from memory not wanting it quite so high. Our girls were little then, and I had to hold them up to see the layout. (Shows how long ago it was-the youngest is now 27). Since we don't have any grandchildren, I decided to build this layout to suit me, and will do whatever when the time comes that we have grandchildren. I know my wife will give me space for age appropriate trains if the time comes.