perfect middle

The maximum contrast is black on white: 21∶1 (and vice versa).
Regular text requires a contrast of 4.5∶1

This means there's a set of colours that can be used on both black and white. A colour that is between 4.5∶1 and ±4.66:1 on white, is also usable on black.
√21∶1 (±4,58∶1) is the perfect middle