I've done this before. It may not be the best way, but what I did was simply rotate the image using the 'rotate layer' tool. You can define the exact angle of rotation. Then all you have to do is draw a horizontal or vertical line using the guides.
As with each rotation, the quality gets worse, I would put a horizontal or vertical line on a new layer, rotate that layer by the desired angle and then merge that layer down to the original layer.
Thus nothing is rotated multiple times.
Andreas _______________________________________________ Gimp-user mailing list Gimpfirstname.lastname@example.org http://lists.xcf.berkeley.edu/mailman/listinfo/gimp-user