Cutting angles accurately
Building a playhouse for my kids, I had occasion to cut a forty degree angle in a piece of wibbly wobbly plastic. You know, the corrugated kind you put on greenhouses. I could have gotten out a protractor, marked 40º and extended it with a straightedge, but I suspect it could be off by a degree or two by the time the line extended to where I needed it to go. Instead, I measured one side of the triangle I would be removing from the sheet, did a little triggety-trig and came up with another side, which I could mark to the nearest 1/8-inch, which represents an accuracy of less than half a degree. Because playhouses need to be precise.