I thought about this a bit last night. I think that the best height for the top of the barrel from the ground, with the rifle standing straight up, is the height at which the arc of my right arm allows me to insert and remove the ramrod and pack the charge without having to adjust my grip on my ramrod. Belt buckle level seems about the minimum height for the top of the barrel, while bottom of my ribcage seems about the max comfortable height. I am 6'1"+- an inch with a 36" inseam on jeans, so my perfect barrel height would vary some from people of different heights, much like the height of our perspective dream flatheaded, midget waitresses/servers/hookers.
Dead Bears, the only good kind.