I'd put the drop the same as my mostest favorite rifle. 
That would be the smart thing to do for sure, Hank... However, my .54 half stock Plains rifle I built 10 years or so ago has a 4 inch drop and it's the most accurate 25 and 50 yard rifle I ever built...
It comes right up to the shoulder and at 25 yards and you'll hit 10's and X's all day long (or at least 'till you get worn out lifting that 10-1/2 pounds...) Then at 50 yards you just aim what you perceive to be center X, and just minutely shade ever so little high and you can live in the 10 Ring all day long,,, I kid you not!
The other thing is that the front sight is 5/32nds off the top flat - while the rear sight is 1/4 inch off the top flat, so it don't take long to get heat mirage with those 90 grain loads of 2fg. But it's truly the most accurate muzzle loading rifle I ever built, but I have to think 5 shot matches before this Plains Rifle gets about a 10 minute break to cool down on a summer day.
Now ol' Candle Snuffer comes in with a 3 inch drop and 1/4" tall sights - but being a .45 and shooting 55 grains of 3fg - I have never found heat mirage to be an issue with ol' Snuffer...
So I'm a thinking since from the top of my shoulder to the center of my eye measures 7-1/8th inches, perhaps a 3-9/16" drop (make it 3-5/8" drop) should play in about right.
I'm going to have to wrestle with this a'while.

Oh as a side note; measuring from the top of the shoulder to the center of one's eye as they stand straight - I have found puts them on a better "shoulder" sight plane... You just divide that measurement by 2 as I did above. It actually does work;
top of shoulder to center of eye measures 7-1/8th inches = 3-9/16 drop. 