Rotary setup mm per rotation value

I am having troubles working out the mm per rotation value for my rotary. The value seems to change for each sized object. Is there a formula to work this value out easily?
Cheers,
Dan

Chuck or roller type rotary?

Roller style. Have got the diameter of the rollers and job just sure how to work out the rotation mm and if it changes depending on the diameter of the job.

Here is a good post in the Epilog site http://support.epiloglaser.com/article/8205/30191/How-to-Set-Up-Artwork-for-Engraving-with-the-Rotary-Attachment

BONUS TIP

Testing Sample
In order not to waste new products, keep 1 product as A testing sample. Test it, then use permanent ink to paint it, and then adjust the parameters if needed. After you are sure it is good to mark, then mark on the good products for customers. Always keep the testing sample and pen by hand, you can use them as many times as you want.

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