Hi there. Don't know if you have gotten an answer for this but if you are still interested in my two bits...
I'm a little confused on the wording of your problem, but I think you are saying that when you are doing this stretch, you are not able to straighten your leg completely as you were standing.
If you can keep your leg completely straightened then you need to stretch your hamstrings more, (yea, i know, it's tight behind the knee).
If you need more help, look over your post and give me more information, and make it more clear.
I too, use the Kurz method.
There are two sides to every story, choose your side.