This script was imported directly from Fandom so it definitely should work on Fandom: https://battlecats.miraheze.org/wiki/MediaWiki:Common.js/StatCalculator.js

This does the input box and dynamic calculation somewhere, which I think is the hard part of the script. as for the percentage between the values, you could probably get it from ChatGPT lol

Here's another version I got by instructing ChatGPT, in case you're interested. It's not perfect but it would provide a starting point so you can tell what you need changed. It works when the page has an element in the form of <span class="percentage-calculator"></span> to host the calculator inside.

(function createPercentageCalculator() {
    // Function to calculate percentage difference
    function calculatePercentageDifference(value1, value2) {
        var num1 = parseFloat(value1);
        var num2 = parseFloat(value2);
        if (isNaN(num1) || isNaN(num2) || num2 === 0) {
            return '0.00%';
        }
        var difference = Math.abs((num1 - num2) / ((num1 + num2) / 2)) * 100;
        return difference.toFixed(2) + '%';
    }

    // Create input fields and output element
    var container = document.querySelector('.percentage-calculator');
    if (!container) return;

    var input1 = document.createElement('input');
    input1.type = 'number';
    input1.placeholder = 'Enter first number';
    
    var input2 = document.createElement('input');
    input2.type = 'number';
    input2.placeholder = 'Enter second number';
    
    var output = document.createElement('div');
    output.innerText = '0.00%';

    // Append elements to the container
    container.appendChild(input1);
    container.appendChild(input2);
    container.appendChild(output);

    // Update output when input changes
    function updateOutput() {
        output.innerText = calculatePercentageDifference(input1.value, input2.value);
    }

    input1.addEventListener('input', updateOutput);
    input2.addEventListener('input', updateOutput);
})();