Length of Pull is a bit of a misnomer, it's probably OK when applied to shotguns when the gun is actually pulled into the shoulder but not really the best way to describe what is required for air or smallbore rifle shooting (Note: My background is in Smallbore 3P and 10m Air). Butt Length is probably a better term to use, but alas it is not as simple as that.
The optimum Butt Length is one that enables a straight line to be assumed from elbow to trigger finger with no deflection of the wrist, vertically or laterally. This is in order to facilitate consistent accurate triggering. The "Crook of the Arm" method is an excellent guide to the required length; FOR THE STANDING POSITION ONLY. For Kneeling (ISSF Rules) it will be shorter and for Prone shorter still.
However Butt Length cannot be taken in isolation, the height of the Butt Plate will also have a major effect. This will normally be highest for Prone, lowest for Standing and somewhere in between for Kneeling. The objective here is that the head drops naturally in a relaxed position directly behind the sight. This of course generates a further couple of problems (I warned you it wasn't simple!): cheek piece height and eye relief. For HFT you can probably compromise on the former, but not the latter.
So where does it leave you? Well RobF is spot on:
going for the low hanging fruit
Optimise for the position that is going to bring the most benefit is terms of points and compromise on the others. You may find that you need a few tweeks to find the best all round settings, but when doing this make only one change at a time. Make a note of what you have changed and then thoroughly evaluate its effect before adopting or abandoning it and moving on to something else. Accurate record keeping and proper assessment of the results along with a methodical approach are the key to success.
Avoid at all costs "I'll try this (or that)" to see if it makes a difference. Make a change only to seek a specific outcome. Finally, watch what other shooters do but don't just try something because someone else does it. It may work for them but not necessarily for you.