The reason you see my 6 inch ruler on top of the paper is because I wanted the sample to fit the lined piece of paper I grabbed. 
(Half scale for a 12 inch block) I actually use my large 15 inch square and use painter's tape at the size block I want. I lay my big ruler over the block to see where I want the wonky to be, cut the 2 exposed sides, flip the ruler, lining up the cut sides with the tape and cut the 2 remaining sides.
This method does leave you with the edges of the block not on the straight of grain so handle gently and use starch.
Last edited by Tartan; 03-08-2013 at 06:14 AM.