RR - no, you are not "wrong." However, it is handier to measure to the (front) trigger. Balance is used to determine how the weight is proportioned between the hands. The trigger is to one place you must for sure touch. So, we have an immediate reference for hand placement by using the trigger. You can reference anywhere you want on the gun, but it only adds an unnecessary step in getting to hand reference. Hand to hinge pin/breech face/trigger guard/etc. is not fixed and adds error to comparing balance measurements among guns. Check this for yourself by measuring trigger to hinge pin and trigger to breech face on several guns.
