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.