Martin I have a piece of 1" galvanised water pipe lays across my yard in a small trench after marking wit string and two tent pegs using exactly this method and a T bar I welded up out of 40mm x 3mm flat bar which intersects to give me accurate positioning of the rear legs of my tripod. Only till I eventually get my pier in but it means I can take my astro trac or mount out any night and be close enough for my purposes. The one mention which may or may not show in Martins diagram is you want to do it on level ground otherwise the shadow will twist and be off a little. In my case with a sloping yard I set up some marine ply and levelled it first to get the suns line at precisely solar noo0n for my precise latitude and level the ply for better accuracy.
Your diagram may mention this point, I'm just full of sleeping medication and can't read it at the moment.
Solar noon is not midday and I recently came across a site which allows you to punch in your address and print out a solar noon chart for the entire year, I;ll see if I can find the bookmark tomorrow. and post a link.
Found it:
http://www.solar-noon.com/