I always do what fjn said and draw the arch full size on a board. You will need a beam compass,a long straight edge and a pair of spring loaded dividers.
When you have cut them all you can set them out on the board and check they all fit in.
yeah, what he said.A huge + of doing it the way Stuart & I mention is as follows. The saw man can start cutting the brick long before they are needed. If you wait until all masons hit the window or door heads you have everyone sending cuts to the saw man at once,he cannot keep up and production almost comes to a complete stop. That is very expensive,having a bunch of guys standing up waiting for cuts.