Personally, I prefer making arches by creating a cylinder (with either 8, 16 or 32 sides depending on how smooth you want the arch to be) and then lining it up in the "doorway" where you want the arch. Then I make the arch by hand out of blocks using the cylinder as a guide, before deleting the cylinder.
I find the actual arch tool to be a pain in the butt!
I made an image which might help you: