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.

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.

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.