clamp() flui (relative) value can be computed using two targeted breakpoints, at which you want your two fixed vakues to be assigned - exactly as you do it with media queries. Example: 15px until 600px - the fluid values - the 18px from 1280px. It's just about maths. Take a look at my article about clamp calculation here (in french, use FireFox or Safari translate function if you don't speak french) and my clamp generator here if you want a clean and speed solution.