How to use up scrap paper

I have a big-ass folder of scraps because I can't bear to throw anything away that might someday be of use. I know this circle can be cut from this triangular corner, but how do I make sure I get it right the first (and only) time?

I lay down a used piece of paper (I salvage the papers from work that have been printed on too), then cut my shape. I cover the circle with my scrap and secure it with removable tape. Now when I cut again, I KNOW it will be 100% where I want. You can see I had just about 2mm margin of error.

Let me know if this method worked for you too!