Tube Bending - creating a coil with multipe 180 degree bends

I am trying to fabricate an "S" coil out of 1/4 in copper tubing. It will have 3 or 4 180 degree bends and I would like it to be approximately 8 inches from bend tip to bend tip.

I have the typical hand bender with the 90, and R reference markings. I have no issue laying out 90 degree bends etc. but I can't figure out how to mark and layout the tubing to create this coil.

Can someone point me to a reference of provide some guidance?

Attached is a rough sketch of what I am trying to accomplish



Thanks,
David

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