The regulator piston is pushed to the left by the regulator spring and pushed to the right by the higher pressure in the regulator chamber. The regulator piston can only move so far to the right before it runs into a stop. The force of the regulator spring can be increased by moving the regulator nut to the right, compressing the spring. Lets say Zr.x0 is the value of Zr.x when the regulator piston is against the stop and Zr. H is the distance the regulator nut compresses the spring beyond its relaxed length when the regulator piston is against the stop.
H is the number of turns per inch of the regulator nut.
When the spring is compressed, it is compressed from the left Zr. H inches by the regulator nut and Zr.x0-Zr.x by the pressure force. The position of the on-off pin is controlled by the sear which is in turn controlled by the trigger. It is assumed that the bottom of the on-off pin is connected to the sear but actually the regulator pressure pushes it against the sear. If the trigger is released, the sear is tilted at zero degrees.