I hate the WCAG's formula and ratios just because it's needlessly complex and pointless and could be done a million times simpler using luminance, which is what I use... the formula from the usability guidelines Apple, M$ and IBM got together and wrote circa the move to color at the end of the 80's. (windows 2/OS 2/MacOS 4 era)
L = 0.3*R + 0.59*G + 0.11*B
Which is the PROPER luminance calculation for projected light as found in the old usability guides; is hardware encoded into all VGA adapters, and can be found in books like Ferrara's "Programmers guide to the EGA/VGA cards"
Also this is another place photoshop sucks since it uses the reflective formula for luminance instead (Y = 0.2126*R + 0.7152*G + 0.0722*B which is actually meaningless outside a CMY colorspace as it overemphasizes green)
Using the proper formula for displays I consider 50% difference the minimum and 75% or higher the ideal. side effect of this formula is the color blind will never have issues with the results either.
For example, #555 is 85,85,85, so that's 85... white is 255... so that's 66%, entirely acceptable. Let's plug in a color combination we know sucks, true red (255,0,0) on true blue (0,0,255)...
255,0,0 = 76.5 == 30%
0,0,255 = 28.05 == 11%
Difference of only 19% across the spectrum means effectively illegible. How about the green and magenta which give the color blind fits?
0,255,0 = 59%
255,0,255 = 41%
well that's a no brainer at only 8% difference.
It's so much simpler to just say "take your two colors, take 30% of the red, 59% of the green, 11% of the blue, add them together, and if the values for the two colors are less than 50%, it's illegible, and if it's less than 75% it could be problematic" than the convoluted mess the W3C tried to foist on us.