My draw length marks are from the top of the tiller tree 2X4, which is the bottom of the handle when the bow is sitting on the tiller tree. I always measure draw length from the back of the bow, so when I'm tillering out a bow, I have to account for the thickness of the grip. So, if I want to tiller a bow to 27" and I have a 1" thick grip (like on a typical ELB), I actually only pull the bow to 26" on the tiller to account for the grip thickness.
For every bow I tiller, I have to remember to account for whatever the thickness of the grip is. If you tiller your bows "upside down" like some of our brothers in the UK, you don't have to worry about this.