[glend (Glen)]

Allan your assumptions are correct, but so are the warnings received. Your nice big shed is going to have a good magnetic field effect on a compass, but even a belt buckle can skew the compass. When I was laying out my observatory i used a builders level and compass to paint a point on opposing boundaries where I drove stakes and ran a day glow brickie string line on the 12 degree declination line to mark true north. A laser is a great way to mark straight lines and is unaffected by wind or magnetic fields as long as your sighting location is unaffected. Have fun.