I have the Parf original version. The edges of the Parf original ruler weren't necessarily parallel to the line of holes. When I made my first benchtop, the holes were slightly out of square due to this. As I recall, I have read of other owners of the original Parf ruler having experienced this.
To compensate, I locate the ruler on the worktop where I want the first hole to be, drill the hole, and insert one 3mm pin. Then I use an adjustable to square to determine the distance from the edge of the benchtop to the edge of this inserted pin. Then I place another 3mm pin in the other end of ruler and use the square to make sure that I am getting the same distance from edge of the benchtop to the edge of the second pin in the ruler, adjust the ruler, if necessary, and then drill the second hole. From that point on, I follow the instructions. The Par II version may not have this issue but you may want to verify.