I would get the dish pointing at the apex of its travel. Then rotate the mount on the pole to find 58. Treat the mount as a pure AZ/EL mount. Mark the pole with a reference mark to the mount. Repeat the aim for 83w and make another mark on the pole.
Now, you have two marks, one for 58w and one for 83w. Make another mark on the pole that's measured to be proportionally equal to your exact longitude. Rotate the mount to this reference mark; now you are aligned to your true South. Lock the mount down there and never move it again.
Ensure your mounts rotational axis is inclined to your latitude, or a bit more if your doing modified polar.
I would expect the +/-10 skew to not be that detrimental, so leave the lnb at 0 skew.
Getting the dish oriented @ true South is most of the battle, then everything else kinda falls into place.
Anyone disagree with my skew comment?