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The eye relief of an ortho runs at about 80% of its focal length. So the 18 should have an eye relief of about 15mm. The beauty of using the Barlow is it will increase that a bit as well as tidying up the edge of field softness that the BCO's have as well.
As far as the 6mm ER in the review, maybe that's measured from the top of the winged eye guard, which sits above the eye lens. You could take the eye guard off to get the full ER.
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