Arches were laid using wooden forms. Autocad allowed us to figure angles on block but there were other variables we had to work through. Thinset that is used to bond block unlike mortar is uniform in its thickness and angles must be cut accurately to maintain radius. You can make up variances in your last block but it is better to cut it smaller so as not to apply pressure to sides of arch. Using a slightly thicker thinset joint 1/8&quot; where as 1/16&quot; is typical helps ensure you don't put pressure on sides. You must also ensure you come up plumb with both sides so blocks face each other evenly or you will have to shape the last block. Using a string from corners and an 8' level we were able to stay fairly plumb.