NAVFAC DM 7.02 (formally 7.2 I believe) has a design method for guy anchors in soil. Check Figure 18. It is copywrited and I couldn't fine any free copies of it these days though.
Your threshold/allowable settlement will come from some structural parameters. Here in Canada, the typical assumption is 25 mm. Differential settlement, in my mind, is more important than overall settlement though.
