The three point method is about all you can do to stretch it.  It is not an exact science, but it will give you an idea in the field what is there.  

I did a field last spring that was far from square with lots of angled fence lines.  My cousin tiled a bunch in the 80's and did a good job of drawing scaled maps, but there not perfect.  He had a measuring device hooked into the speedometer on his pickup to measure the lines.  With that field, I had to kind of scale it  in sections.  I uploaded it 3 different times with the points relevant to certain parts of the field.  I used a different map depending upon what part of the field I was working on.