Use a center point that is the correct diameter to move the radius of your spiral by the desired distance per revolution. Pi X D, 3.14 times the diameter, give you the distance increment per full revolution. If you wanted one inch between the lines of the spiral, you would divide that one inch by 3.14 and use that number for the diameter. I don't know how accurate this needs to be, but a piece or string wrapping around a center point of the correct diameter with the other end attached to a pencil or scribe would certainly do the job for ordinary work.