As Eric indicated, there can be a good bit of variation, and you should always use your original reference firearm to answer these sorts of questions. That said, there is a general trend in the amount of sideplate, or trigger guard for that matter, that stands proud of the wood in that early rifles tended to have the most side plate showing and the late flint and percussion guns the least. It is sometimes hard to tell given wood shrinkage over time, particularly, if the side plate is sitting up just a bit. In that case, you might compare with the patchbox or nose piece. I should also note that the angle of the bevel can change over time and between schools. Generally, this is more of an issue with the side plates with the most metal showing.