My suggested test used the gray for background, with black text or white text over it. I can't trust the math, because I don't know how they derived it. Well t

My suggested test used the gray for background, with black text or white text over it. I can't trust the math, because I don't know how they derived it. Well the conversion from sRGB to linear RGB and from there to luminance is straightforward and standard, but the formula for contrast is a real mystery. Where'd those magic 0.05 constants come from? And does it really not make a difference if it's light on dark vs. dark on light?