This code is a pice of 💩and should not be used unless you are planing to fix it up a bit.

range-font-size-scaler.js

// Input must be sorted in asc order
const input = [3, 8, 20, 25];
let output = [];
const minmax = [10, 30];

let distances = [];

for (let i = 0; i < input.length; i++) {
    if ((i + 1) < input.length) {
        if (input[i] > input[i + 1]) {
            distances.push(input[i] - input[i + 1]);
        } else {
            distances.push(input[i + 1] - input[i]);
        }
    }
}

const stepSize = ((minmax[1] - minmax[0]) / distances.reduce((a, b) => a + b, 0));

for (let i = 0; i < input.length; i++) {
    if (i === 0) {
        output.push(minmax[0]);
    } else {
        let sumLowerSteps = 0;
        for (let d = i; d > 0; d--) {
            sumLowerSteps += distances[d - 1];
        }
        output.push((sumLowerSteps * stepSize) + minmax[0]);
    }
}

console.log('input', input);
console.log('output', output);