Well I gave the 12inch block a go, works well, but it still took me more than half an hour to mostly remove the tool marks from the ironbark. Much more effective than my shorter blocks. Since I only vaguely remember making them I was relived to confirm that they were both the correct radius. The amount of work required to remove the tool marks makes me pretty sure shimming the router to correct the angle of the bit to the surface is worth the time, especially with the 12 inch jig, cause I have 4 boards to do.