LOP is measured from the trigger to the centre of the butt plate. Simple!
Drop should be sufficient to allow easy alignment, with your eye naturally centred and slightly over the rib. If you are like me you will likely shoot best if you should see just a bit of the muzzle and of course the bead.
Proper fit begins with a proper gun mount. it is amazing how many people do not know how to mount a shotgun properly. ( they mount and aim a shotgun like a rifle) Take a lesson from a good sporting clays shooting coach, or buy some videos. Do not learn from competition trap or american style skeet shooters who shoot with a pre-mounted gun! I learned a lot from videos produced by Bryand Bulinski and also some by the Holland and Holland shooting school.
All things being equal, a straight comb will smack your cheek less than one with a lot of drop between the nose and heel of the comb.
I am 6' tall, average build, and like 14-3/4" LOP, 1-3/8" drop at front of comb, 2-1/4" - 2-3/8" drop at heel, and about 1/4" cast off. Your "perfect" measurements will of course vary.