If it's a Carole Roberts design, she rounds the angle UP.
18.4 deg is for a 1/4" ring on a 3/4" board. If you are measuring 15 degrees when you eyeball it, you should think about why it is that far off. 15 degrees would be a 0.20" ring on a 0.75" board.
Another trick Carole uses, that I found really works well, is to use only the outer ring from the pattern. After the outer ring is cut out, use the bottom inner edge that ring to mark the next ring to be cut, and keep working your way in doing that. That way, if you wiggled a little in your any cut, you will follow that wiggle for the subsequent cuts. You get better alignment and the angle doesn't much matter.