It works best when you can use helper springs rather than the stiff regular springs, but no biggie.
What you do is install the checker like a regular rod. You adjust it so the rocker tip is centered on top of the valve but color the tip with a Sharpie before you do. Get it adjusted, then you rotate the engine 2-4 times. Remove the rocker and check the wear pattern on top of the valve tip. If the mark is more towards the intake side, the rod needs to be longer. If it's towards the header side, it's too long. Adjust the checker until you can rotate the engine and have the wear pattern directly in the center of the valve tip. Then you measure the length of the checker with a dial gauge and that's the exact measurement you need.
What you do is install the checker like a regular rod. You adjust it so the rocker tip is centered on top of the valve but color the tip with a Sharpie before you do. Get it adjusted, then you rotate the engine 2-4 times. Remove the rocker and check the wear pattern on top of the valve tip. If the mark is more towards the intake side, the rod needs to be longer. If it's towards the header side, it's too long. Adjust the checker until you can rotate the engine and have the wear pattern directly in the center of the valve tip. Then you measure the length of the checker with a dial gauge and that's the exact measurement you need.