It can be quite tricky to obtain the principal strain state from the three gauge strains in a 45-degree rosette. Numerous textbooks provide equations for the principal normal strains, and generally these "just work". However, the angle of the principal strain state is often quite ambiguous. It's often difficult to figure out whether the angle is clockwise or anti-clockwise, which gauge it is relative to, and to which of the two principal normal strains it refers.
I have recently done some work on this problem in the context of measuring strains on the equine hoof wall. I have written a short description of the Analysis of Strains from a 45-Degree Rosette (PDF file; 47.9 KB). Please feel free to use this document (with an appropriate link to my blog), and please contact me regarding any corrections.