You are going to have to compromise. The objective is to have the wrist in a straight line to the trigger hand, the hand in a comfortable position on the stock allowing the trigger finger to be correctly positioned on the blade. The crook of the arm method works very well, for standing only. For kneeling you require a shorter butt and for prone a shorter one still. Take a look at the slide show here
. The variation between prone and kneeling can be quite small, but the extended length for standing is fairly easy to see.
There is no formula or easy way of working out the length required for prone or kneeling. It has to be done by experiment and observation.