I was successful in using one rect() and moved it using the mouse cursor to control the servo.
The rects that you create should NOT move with the mouse. The rect() function takes 4 arguments - one corner, a length, and a width. They are actually shown only to provide a visual reference.
If you have a rect() that starts at 0,0, has a width of 100 and a height of 80, and the mouse is at 200, 60, it is clear that the mouse pointer is not in the rectangle, right?
If you have two rects, one that starts at (0, 0) while the other starts at (100, 0), each having a width of 100 and a height of 80, and the mouse is at 180, 60, you can determine that the mouse is not in the first rectangle, and that it is in the second rectangle, right?
You can determine that the mouse is 80 units to the right of and 60 units up (or down) from the origin, right? So, given a mouse position of 180, 60, you can determine that the values that you want to send are 2, 80, and 60, to move the second pair of servos to 80, 60.
It should be clear, then, that rectangles of 180 by 180 are the ideal size, and that you need one per pair of servos.
Can you see how to compute which rectangle the mouse is in? Can you see how to compute where in the rectangle the mouse is?