How do I determine correct pushrod length?

Once again, the instruction sheet is your best friend, so refer to it often.

Place the pushrod length checker into a lifter and install a rocker arm assembly. Be sure the cam is rotated to the base circle. Seat the bottom of the adjuster screw up against the recess in the rocker arm and turn the adjuster screw clockwise one full turn down. This is the initial adjuster position. Adjust the pushrod length checker tool to the proper length, remove from the engine, and measure its overall length.