HTML is bones. CSS is paint. JavaScript is the hired hand that does the arithmetic so you don't have to.
Add two columns to the table: Start lbs and Now lbs, plus an empty Gain column. Put the numbers on the row as data-start and data-current.
Live result
Step 1 — mark the rows
Black Gold FW-01 Texas Longhorn 1100 1280 —
Do that for all four. Then a button under the table:
Step 2 — script.js
New file in the same folder. Link it at the bottom of body:
document.getElementById("calc-btn").addEventListener("click", function () {
document.querySelectorAll("tr[data-start]").forEach(function (row) {
var start = Number(row.getAttribute("data-start"));
var current = Number(row.getAttribute("data-current"));
row.querySelector(".gain").textContent = (current - start) + " lbs";
});
});
Click the button. Gain should fill in. That's the page doing ranch math.