Equidistant positioning

[agamemnus]

I'm trying another tack on doing the "bumps" I've mentioned in a previous thread, though with somewhat different geometric results.

I'm aiming to do follow-me on this to create rings. The problem, however, is that I would like each ring to be equidistant from each other ring so that all 4 curves (blue) are of equal length. Is there a plugin that can figure this out for me or do I need to put on my thinking cap and do some somewhat complicated maths to solve this one?

I've attached a file with the same geometry as that emphasized in the image.

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rings.skp