This is what works for me as far as "drop at the comb" is concerned.
Once you get the Lenght of Pull to where you can comfortably and easily shoulder it, Drop at the Comb is the most important in my view.
It only works on guns with a middle bead - - -
Close your eyes and shoulder the gun, when you look down the barrel (without moving your head on the comb) the two beads should appear as a figure 8, with perhaps a slight bit of space between the loops of the 8.
For me - - - if the beads line up the gun will shoot low, if there is too much space between the beads the gun will shoot high.
The true test of fit is the patterning board. Put a bright post it note in the center of the board.
Quickly mount and shoot the gun while focusing on the post it note - - - if your pattern is not fairly well centered your gun does not fit.
Be sure to shoot both barrels of doubleguns as the barrels sometimes will shoot to different spots !
You can raise or lower the point of impact by adjusting comb height - - -
Moving the impact point left or right is above my paygrade