I'm 5'4 & 1/2" and have always had trouble with lop on rifles. One's height is not always a good indication of lop as some people have shorter arms or wider/narrower shoulders, etc. But it is a good place to start. When you get a custom rifle built you normally measure your lop for the gun. Since modern rifles have lop around 14" I've always had trouble seeing through scopes and had to choose them based on whether or not I could get it close enough for a full field of view. Hunting clothes exacerbated the problem; you need a long neck in that situation. Most tall & long limbed shooters can deal quite well with shorter lop (whether they admit it or not) but the reverse is not always so.
Compared to my built rifles, the factory rifles I own are more difficult to shoulder & aim and the toe tends to snag on my jacket/shirt. You can certainly make do with a "one lop fits all" rifle if the piece shoulders and feels okay when you mount it. I've managed that for years. If you mount one with YOUR lop the difference is, however, striking. I wouldn't turn down a rifle I liked just because of lop. Shotguns are fired differently than rifles and handle better with a "longer" lop; shotgun fit is like visiting a tailor. Not so much with rifles.
To measure your individual lop hold your arm straight out to the side at shoulder level. Now bend your forearm 90 degrees so your trigger finger is pointing straight ahead. With a tape, measure the distance from the bend in your elbow to the first joint of your trigger finger. That measurement will be your lop and is independent (mostly) of your height. If you're having a rifle built and it is to be used primarily in cold weather (hunting, etc.), It might be wise to have cut maybe 1/4" shorter to make up for thick clothing.
I find most factory muzzleloaders rather easy to adapt to since they have open sights rather than scopes. Plus, as was mentioned above, you can have the butt shortened if you want.
