Measure the widest point and the longest point and add at least 1/2 to both. I usually add a full inch or inch and a half, it wastes a little more but I don't have to fuss with it so much.
p.s. I must confess, I'm not a professional but I've made 12 mariners compass blocks and a few others. I've also taken a Carol Doak workshop. I hope that's enough. :lol: :lol: :thumbup: