Ergonomic load height for an operator on industrial equipment, like a guy putting parts into a machine that will automatically assemble the parts is 38"-42". This is for someone who is standing. Your hands will be a couple of inches above your waist. The easiest on your back would be a bench that you could sit at for most assembly, and another one that was at 38"-42" inches. Sounds like the easiest solution is a bench at 38"-42" and a height adjustable task or drafting chair on wheels, so you could sit or stand. The drafting chairs have a ton of height adjustment. Good luck.