Actual draw length is 1 3/4" in front of the pivot point. So measuring to the back of the bow is an approximation, since riser thickness differs. For most purposes, this doesn't make much difference, but it does emphasize the importance of individual testing to determine such things as proper arrow spine, etc.
Also, if you want a draw weight at your draw length of 45#, and you (think) you draw 27", if you order a bow marked at 47# @ 28", it may or may not come out to be 45# at your draw length, depending on whether you and/or whomever is selling the bow are on the same wavelength as to how drawlength and draw weight was measured.