Contrast ratio is not a good name to describe this concept. A contrast ratio is a decimal number, while a "magic number" is an ordinal value. Also, a contrast ratio has a clear definition from WCAG perspective, and it would be even more confusing to use the same term for two very related topics (magic numbers are based on WCAG contrast ratios).
"Magic number" is not good either, for the reasons you've mentioned. But while doing my research for this topic, I couldn't find any better alternative, nor did I want to come up with my own term. I think this topic is niche enough, so we don't have to overthink it.
"Magic number" is not good either, for the reasons you've mentioned. But while doing my research for this topic, I couldn't find any better alternative, nor did I want to come up with my own term. I think this topic is niche enough, so we don't have to overthink it.