The most important thing to remember is that they taper along the length, so the end nearest the handle is the largest. That means that a long hole will also be tapered.
However, the more important point is to measure at the largest end, and to cut through to the same point. On thick metal, use it from both sides if the slight taper is likely to cause a problem, or cut right through.
When adjusting, always fully tighten the adjusting/locking screws so that the cutters are firmly fixed, or they will break. (Here speaks the voice of experience!)