Thanks for the replies everyone. Some of you have said that you lay out the draw on the tenon without pre-assembly. How do you make sure that you are precise enough considering that if your drilling is just a fraction off then it might draw the other way.

Sometimes when I read posts about timberframing (especially mine) they can get confusing.

When we drawbore. We pre-fit and use a peg of the appropriate size slightly sanded around the circumference (thanks Joel and Gabel!) with a pin offset from center the distance we want to drawbore. If you lay-out without fitting up how can you make sure your drilling is precise enough to draw the shoulder?