Put your boot on your foot, buckle the upper cuff, flex into it to seat your heel in the heel pocket. Take a hammer and gently tap on the side of the boot till you feel the ball of your foot... that's it. Make a mark, masking tape helps for this.
Take your liner out of the shell, get an assistant to help hold the shell open, put lipstick on the ball of your foot. Slide your foot into the shell, position your foot centered in the boot, touch your foot to the shell. Get help holding your boot open and carefully (don't smear lipstick everywhere) remove your foot. transfer mark from inside the shell to the outside by holding a finger inside and one outside, line-'em up. Make a mark.
I'm sure you are looking for something a bit more 'scientific', but that's pretty much it.