Be reincarnated as a women, seriously they have a unique ability to get there knees apart and splay there feet outward making a tripod between there butt & 2 opposed feet. Ok unrealistic
If the back strap in high and under your shoulder blades your going to slump and your lower back will SCREAM !! Found when back strap is above hips and in the small of your back has support more comfortable and you can relax as if in a chair.
The leg straps ( pending how you lower legs get positioned ) generally in the upper 1/3 of lower leg ( Just under knee to shy of mid thigh )
Ideally you want to pay attention to your center of mass on your bumm bag and not start leaning as to have all the pressure on one butt cheek / bone about to fall over.
Play with position holding your gun WITHOUT A SCOPE ATTACHED and figure out where gun needs to be to get a stable and semi-centered position over your bag.
That done, address scope rings & mounting of optics to allow the position and a good eye relief.