To measure Drop, use a straight edge( ruler) on top of the barrel flat and extend the edge out over the length of the buttstock. There is a drop at comb, which is measured from that extended line on the bottom of the straight edge, to the top of the comb at it forward most point, where the comb drops down to the wrist of the stock, and then the Drop at Heel. Heel is the top back of the buttstock behind the comb. Its as far back from the barrel as you can get.
Think of the buttstock as a human foot when the stock is stood on the buttplate. The part of the butt that is at the bottom of the stock, that angles upward to the trigger, is the " Toe". The part of the butt at the top is the "Heel", just like a human foot.
If you are making a gun that will be shot off-hand, the drop at heel needs to be deep enough to allow your eye to easily align with the sights on your barrel. The height of the comb also deals with this concern. Since your eye is most likely to be fairly close to the front of the comb, the distance or " drop at comb" should be set so that you eye centers the rear sight without having to lift your head up off the stock, or pull the stock into your face to get down close enough to the stock so that you can see the sights at all. I want off-hand shooting with iron sights to determine these dimensions so that your head is held erect, against the side of the stock( Cast-off, or Cast-on[for lefties], and with your eye focusing through the center of your eye, rather than looking out of the top of your orbit. This often requires a Drop-At-Heel of 4" or more, on tall shooters. Anything shorter, and the shooter has to " Crawl the stock" and look out from under his eyebrows to see his sights. That produces eye fatigue quickly, and is not conducive to good shooting.
The amount of (down)Pitch given the stock can sometimes help fit a stock to a shooter with a long neck, or with wide shoulders, where making the stock with a lot of Drop at heel makes for an uncomfortable amount of felt recoil.