Accessibility guidelines say body text needs a 4.5:1 contrast ratio against its background. That number means nothing until you see it.
The intuition
4.5:1 is roughly "charcoal on cream." 7:1 is "nearly black on nearly white." Anything under 3:1 is decoration, not text.
If you have to squint to decide whether a pairing passes, it doesn't.
Every Digisperse palette guide includes a pre-computed pairing table, so you never have to run the numbers yourself.