Myself I find the GM fascinating and use it often
I think where a lot of folks run into issues is they wrap themselves into the math of the GM and not the application .
You don’t need to set around trying to calculate a 3 to 5 . Just make you a couple sets of dividers set at 3 to 5 and then be done with it ..
The other thing I find is that a lot of folks simply point out that X isn’t in proportion with Y when using the GM . However they never consider that maybe that’s because X is not supposed to be in proportion to Y but instead should be in proportion with Z.
As such you have to know where to start and not just pick a point
Thus you get what Mr. Getz alluded to . IE it should work out to some other point . IE the side plate may fit correctly when in comparison to a given point in the curve of the stock . Maybe the point of an inlay or that start of checkering . That measurement then in turn , will define another measurement
I also do not subscribe to the idea that everything was measured .
Myself I lean more to the opinion that probably it was more of a natural happening of the trained eye . As such it probably wasn’t really something that was though all that much about . Instead it just happened .
I think also all to often we forget that the GM is used for layout from start to finish .
So if you have not used it all the way through your build . IE from the drawing of your plans right through making your decisions on hardware, your probably going to be off Or find the need to fug things ,.
But regardless of what some folks view may or may not be concerning the GM , it still cannot be discounted as a fundamental base.
So here is my advice to you .
You either need to simply use your eye and apply what you feel looks good . OR if your going to use the GM you have to follow through with its use . Which may very well mean you have to modify or make your own side plate .
IE if your using the GM for your layout and you lock choice works . Then find or build a side plate that is in the same proportion as your lock plate .