What I did
I cant actually tell you the height I used, as it was 25 years ago..but here is an old picture of how I did mine that will give you an idea. I tied 4x6 posts to my basement rafters and epoxied them to the floor, then attached aluminum angles to the post in which I rested/clamped the spars. i set it up so both wings were being built simultaneously and could perform the same tasks on each wing. i was comfortably able to shoot the L/H skin rivets alone by reaching up in through the lightening holes to hold the bucking bar while shooting. The height from the floor is important for overall comfort. You can see in the picture I used a couple "kickstands" on the rear spar to keep everything level. With the plumb bobs installed per plans, it was very easy to keep everything perfectly true. My assistant in the background was too young to buck rivets until later in the project, but he kept me motivated.