[ChrisH]

Good question. Even the official drawings for the real field do not have this dimension. At least I couldn't find it.

They do have a dimension for a hole in a vertical pipe that is near the centerline of the lower horizontal pipe. The hole is 8.5" below the centerline of the top pipe. The centerline of the lower pipe seems to be between .25" and .5"above the hole. This would put the lower horizontal pipe 8-8.25" below the upper pipe.

For most purposes this should be close enough. You aren't supposed to hang on the horizontal bars on that side anyway. The interface with the ramp is the only reason to sweat it