I found this quote in an old post:
Ok, when my thighs when parallel to the floor, I actually used a level, my chair height comes in at 17-1/4"ergonomic guidelines say your thighs should be parallel to the floor, likewise your forearms, with little or no flexing of the wrists in either direction. To get those relationships, you will usually need the lowest part of the instrument (back apron, knee lever, whatever) to just clear your knees or thighs.
At 17-1/4 I have 2-1/4" clearance from the top of my right knee to the bottom edge if my guitar.
In order for my arms to be parallel with the floor my guitar would need to be 3-1/4" shorter
Of course I only have 2-1/4 to work with, I could get more if I ditched the cowboy boots but I always go to gigs in boots.
I supposed I could have notches cut where my knees go to get the right height.
Anyway, before I call Roy Thomas about modifying my Pedalmaster I thought I would see if any of you guys have done something similar.
I'm trying to get myself into the correct playing position and I don't expect I will grow anytime soon.