The ‘rotationDiskRadius’ you see refers to the distance from the end of the chain to the center of the bit on the sled, a value that can be set in GrouncControl either by hand or by the auto-calibration process. The value referred to as ‘ChainLength’, shown in red on the diagram, seems to be the value you’re referring to I think.
The calculation you mention hasn’t been integrated into the triangular kinematics code yet, see a post from Bar on the subject. I’ve poked around with the calculation, but haven’t taken time to do any meaningful testing on a live system.