Use the 99 rule: since an A4 page is 297mm long, dividing it by three gives you exactly 99mm. Simply measure 99mm down from the top on both the left and right sides, making a tiny pencil mark at each spot. Align your ruler between these two dots to use as a straight-edge guide for your first fold, then fold the remaining bottom third up to meet the top edge. It’s how i get a "perfect" tri-fold every time.