The ERC code would belong in the Xrotate parameter.  Before adding the code, the Xrotate parameter's paragraph will look like this:

=================

        rotateX xRotate
            {
            name xrot
            initValue 0
            hidden 0
            forceLimits 0
            min -100000
            max 100000
            trackingScale 1
            keys
                {
                static  0
                k  0  0
                }
            interpStyleLocked 0
            }

====================

You'd add the ERC so the paragraph now looks like this:

======================

        rotateX xRotate
            {
            name xrot
            initValue 0
            hidden 0
            forceLimits 0
            min -100000
            max 100000
            trackingScale 1
            keys
                {
                static  0
                k  0  0
                }

            interpStyleLocked 0
            valueOpDeltaAdd
                NO_FIG
                Ball
                xRot
            deltaAddDelta 1.000000
            }

=====================

The NO_FIG is important; the next line (ball) doesn't really matter but has to be there. And the attached file is a script that will give you the correct number to use in deltaAddDelta instead of the 1.000000 in the above text. 

Select the prop, run the script.  It will calculate the prop's diameter and tell you the number for deltaAddDelta.  (I use this all the time when making car models, for the "drive forward" dial.)  The script also works for a body part, as in a wheel.

Download the attached Diameterfinder.py.txt and take off the .txt extension before using.

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