Here is a sketch that I made of the trellis, this should make things a little more clear. I hope to have some more pictures by this weekend, if I can get it finished.
If you do the math...A²+B²=C² on the triangle A=18' (height) B=10' (post to anchor) then the hops cord (C, the hypotonuse) would be just over 20 feet, not bad. If I go farther on the base distance the cord gets longer, but the "weight" seen my the anchors gets higher as the angle becomes sharper. Shouldn't be a problem with this stucture though.