I'm 5' 9" and used 48". I needed a step stool to work on the leading edge a couple of times. Here's what my posts look like. (I may take them down today if I finish the cradle )
I bought an adjustable mechanics stool on casters. It has been a huge help.
I'm also 6'4. My spar's are set at 50", work's out very well especially when working at the bottom. One of those small rolling mechanics stools will really save your back too.
I put my main spar at 42", that left the bottom 13" from the floor. I am just about to put the tanks and leading edge on for good and I havent run into a problem yet. BTW, I am 6' tall.
I am 5 11 and my shoulders are 58.5 inches high. The main spar is at 44 inches and the rear spar is at about 17 inches from the ground. So far is working well for me.
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