This is from the AMO Standards Committee Field Publication FP-3 2000, prepared by Norb Mullaney, Chairman:
"AMO DRAW LENGTH STANDARD
For Manufacturers
Draw length is a specified distance, or the distance at the archer’s full draw, from
the nocking point on the string to the pivot point of the bow grip (or the theoretical
vertical projection of a tangency line to the pivot point parallel to the string)
plus 1 3/4”. Draw length from pivot point shall be designed at DLPP and shall be
called TRUE DRAW LENGTH.
EXAMPLE: 26 1/4” DLPP plus 1 3/4” is the equivalent of 28” draw.
For Dealers and General Use
For practical reasons not requiring precise terms, draw length is the distance, at
the archer’s full draw, from the nocking point on the string to the back of the bow
at the arrow rest.
EXPLANATION: The standard Manufacturers is consistent with the Bow
Weight Standard as related to the pivot point. The DLPP plus 1 3/4” is compatible
to previous concepts of draw length. (See handle illustration.) Draw length
for Dealers and General Use relieves the burden of preciseness not required for
general use and facilitates determining arrow length. THIS STANDARD SUPERSEDES
THE PREVIOUS STANDARD.
AMO"
The pivot point is what most folk call the throat of the grip, which is the low point of the grip. The standard allows for measurement from the nock point at full draw to the back of the bow. I assume they have an arrow with a ruler index on it that you can draw in the bow and determine what the length is at the back of the bow or you can draw your arrow and have someone mark it and then measure the length from the throat of the nock to the mark.
David