The colour range doesn't seem to be quite linear. For instance, setting the variable to 0.5 gives you a grey line of colour #4d4d4d. That's already 70% black.

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It looks like the color you get may not be entirely consistent between computers. On my Aluminum iMac, setting the value to 0.5 give a border with a gray of 29.8%. If I do the same thing on my PowerBook G4 (which never supported translucency anyway), I get 37.3% which is a little lighter than the color you're seeing.

I'm using Apple's DigitalColor Meter utility to get measure the color of the 1 pixel border. So, I guess if one really ones it to be perfect, you just have to play with it. But generally, something around 0.5 or 0.6 seems to end up looking pretty good.

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How bizarre!

But yes, your originally suggested value of 0.63 looks "just right" (to my eyes) on my ageing Powerbook G4. It gives me a grey line that's #616161.

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