I make sure my blank is planed smooth and do any drawing on it. You can always erase. Also, I inlet my barrel and drill my ramrod hole before drawing, then take my measurements from those known variables. For example, when the barrel position is fixed in the blank, I now know the exact location for my touchhole and therefore the exact lock position. Once that is done, I can then establish my length of pull, drop etc. I used to draw them out on paper but somehow for me, it never translated exactly to the blank leading to problems later. I have built several guns over the years without doing any drawing except the bottom and breech of the barrel, location of ramrod hole, location of lock sear, drop and length of pull. Everything else done freehand.