@Nicholas Seward,
Nice looking pair of robot arms!
Tks for the explanation, and for stating that the cam form is circular, so your goal is to calculate a radius.
Roughly to my unaided eye, it looks like the radius of the cam is 20% to 30% of the length of the arm. I have estimated that the cam radius for my 6 inch arms to be 1.5 inches. I'm going with 1.5 inches until I understand the math. Because your arms are using the block and tackle, and the two holes, and a few of your input values are unknown to me, I'm unsure of where I should take my measurements.
I'm wondering if a fixed percentage of any arm length (center to center), could work.?
Edit 1:
Are you are free wheeling the stepper motor, when the arms of the Simpson with springs (block and tackle) lift up? If you are free wheeling, does the timing belt pulley driven arm (like what I have drawn) necessitate reverting back a small portion of the original code, (i.e. uncommenting the original code)?
Edit 2:
Added a picture of a chunky looking arm, it's less than 6 inches long. A 3 inch diameter timing belt pulley is show.
A2