Skin tones has always been a difficult topic to parse. For me, it is a very fascinating one as well because, although I am not very colour blind, I am very colour unaware. My guess is that most professionals have an intuitive sense of skin colour and manipulation.
In general, what I would say is that skin colour, even per individual throughout one’s life, is very diverse. We aren’t even talking about light source and colour constancy challenges. As an example, see https://www.angelicadass.com/humanae-project. That said, ratios and stats are important in the field of computational photography such as detection, identification and machine learning.
From the papers I have read, it is usual good to take a multi-pronged approach. Try using two sets of common ratios in two difference colour spaces.