There are a few things you should take note of immediately when it comes to how a leather shoulder holster fits, as they are more or less instant indicators that it doesn’t fit correctly.
The straps of the shoulder holster should be snug on your shoulders but shouldn’t be tight. They should allow for relatively uninhibited movement of the arms. Obviously, you’re wearing a harness on your shoulders; it’s not as if it will never feel as if it isn’t there. It’s more that the presence of the harness shouldn’t get in the way.
It should neither flop nor constrict; the harness of the shoulder holster should be relatively neutral.
As to the holster itself, the holster should sit in a location that doesn’t cause discomfort or inhibit the motion of the arm. What some people find is that it sits too high under the arm for comfort, almost jutting into the armpit. That isn’t pleasant.
If your leather shoulder holster also includes a magazine carrier or a second holster for use as a double shoulder holster, the same idea applies.
For many people, the ideal ride position for the pistol will be about halfway between the belt and the armpit, so the pistol shouldn’t ideally sit higher than the ribs just below the chest. Granted, what feels comfortable is going to depend on you; you’re going to be the best judge of where the holster feels most comfortable to ride.
Depending on your exact dimensions, your holster may or may not be in this position naturally.
These two aspects of fitment are critical. If the straps aren’t comfortable and don’t move with you, the holster isn’t going to be comfortable to wear.
If the gun rides in an awkward position, the holster is going to be uncomfortable to carry with and will furthermore be awkward to draw from. And if you can’t easily get the gun out if you need to in an emergency, it defeats the purpose of the venture to start with.