What a great idea. I usually set up in the same spot so I have marked it all out on a concrete pad but I often run astronomy nights with the kids I teach at school and I can see your method being very useful (now where did I put that mig welder...). Your true south bearing is about right at 178 degrees (being a science teacher I always go about it the hard way e.g. 1 degree 44" E of due south but hey you can never get lost if you start from first principles
. My reference of 180 was to due south on the comapass not for correction for the mag dec). I use the site below to find out magnetic declination.
http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp