I built mine at 42", but then raised the floor 12" in the aisles, for a net of 30" to view, but 42" to work underneath. I use a rolling office chair for much of my topside work, and just sit on the floor down below.
If I can't reach from the seated position, 30" is easy to lean over.
I also have an upper deck at 72" off the concrete, but only 60" off the raised aisle. A step stool or a ladder are used depending on how far back I need to reach.
