Why not take a lead from Ineos and the published figures for the roof. Static load is 420kg and dynamic 150kg.
That's way different than ⅔rds, more like ⅓rd (35.7%).
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And looking at the Yakima website on the issue, they state "As a general rule, in a static environment, the load rating can be increased by a multiple of three (3)."
So using 3 as a denominator seems OK but the numerator should be 1 not 2.
All this is caveated by the requirement to have any load evenly distributed.
As an aside, and looking at roof rack loads vs mounting arrangements, they offer this:
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Gutter mounts draw the largest reduction in load. I wonder why Ineos, with a clean sheet design went for gutter mounts when it has the choice of not one but two fixed point attachment locations via the roof rails and the roof strips.