A diagram can provide us a much more intuitive understanding of how this math works when applied to a servo motor. Figure 1 illustrates the block diagram of a linear servo PID control system ...

In the setup() function we set the pins were the LDR are connected as INPUTs and position the servo motor at 90° then wait for a 2 seconds before the code execution inside the loop(). In the loop() we ...

The circuit diagrams and code for both types of mouth are all available on the site including videos of the actuated mouth and a gif of the LCD mouth in action.