Do you mean the back of the riser? The back is the side of the limb you can't see when you draw the bow. It's the side facing away from you, and it is the correct side from which to measure your draw length. So if what you're calling the front is in fact the back, then 27 1/2 is your draw length, if you took the measurement using a recurve or longbow.
You should take your measurement with a traditional bow, not a compound. Most folks take a more formal/target stance with a compound, i.e., body is exactly perpendicular to the target rather than turned in toward it a little, straighter upright posture, straight locked bow arm, high wrist grip and perhaps exaggerated back tension. All of these things lead to a one to two inch longer draw using a compound compared to a stick bow.