On my JD Model L/Hercules Engine, model NXB engine,and Wico X Magneto the impulse is supposed to trip when the "DC" mark is at the bottom of the timing hole. Mine trips well after that. I have had to loosen the impulse hold down screws and move the impulse ring clockwise as viewed from the front(gear end) about 2 divisions clockwise to get it to trip at the bottom of the hole. When I got the magnetos from John Brillman( to replace the Wico C ) , the impulse was set at 1 1/2 divisions(about 20* lag)which agrees with the reference chart for Wico X magnetos . It had no trouble starting and running at the 1 1/2 (20* lag) but the impulse was about 1/4 to 1/3 below the timing hole, making the piston well below the timing hole. I am missing something here?,It's been about 40 years since I have done magneto work. What is the correct setting for the impulse ring?