This is the guidelines I use to determine the measurements being discussed.
Now, if you are specifically building say a Bedford rifle it would be impossible to stay true to the architecture with a 3" drop at the heel, but, if you can "jig" the measurements a little you can build a better fitting rifle "from the start".
Might the shooter want to have it adjusted/tweaked? sure, but you can only build to a given set of specs. Any follow on adjustments, provided you produced what was asked for/detailed in the first place, should be on the "buyer", not the builder.
Anyhow, this is what I use:
(the last measurement listed is for Length of Pull - I forgot to identify it)
While I have had criticism of this method before, I only suggest it is a viable option (and not the holy grail).
Using this method on myself I come up with a 2 1/2" drop at the comb, 3 1/2 drop at the heel, 1/4" cast off and 14 1/2" length of pull (I'm 6'1" tall and wear a 34" sleeve in a shirt).
My best handling "gun" is a 20 gauge shotgun. While it has no "cast off", the LOP is 14 1/2", the drop at the comb is 2 1/2" and the drop at the heel is 3 1/2" (and I measured the gun "after" tripping over this formula).
So for "ME", the above method worked near "perfectly". So I absolutely use it, if at all possible, when building a rifle for someone else (which is rare in my case, so have very little anecdotal evidence, beyond my own personal rifles).
Take it for what it's worth.