My experience with Scout The Wonder Dog is that he considers himself a growedazz dog and he'll sleep where he wants to. I generally use a 20 foot long shielded wire cable to fix him to a tree. He carries it in his pack, so the weight (for me) is not a big concern, and he can carry a LOT of his food even with the cable. When the weather is very cold or raining very hard, he chooses to sleep under me on the ground cloth. When its nice he curls up somewhere outside the tarp or if the rain is light, just inside the open end.
One thing to consider, and this is just my opinion, is that the dog might want to choose where she will sleep, and may be the better judge of where is the best place. Scout's primary concern, it seems, is to guard the location. I think that's why he sleeps outside the tarp in good weather and just inside the tarp if its rainy. I think the reason that he sleeps under me when its cold is that is the point where he considers warmth to be more important than guarding. The only real problem I have with giving him this much liberty is that sometimes his cable fouls the guy lines on the tarp. he generally figures out where he can and can't go after a few tries, but I still have to free him up on occasion so that he can learn the lesson.
My ground cloth is a 36" square of DVR. In cold weather I carry a thin CCF pad as a sit pad, and put that down on the ground cover for him to sleep on. The only time he seems to want to sleep on it is when its cold. Since you're using a six foot leash, your dog's choices will be somewhat more limited, which may be more of a curse than a blessing, especially if she wants to sleep 7 feet away from you.
Of course, training will help. If you go out with her every weekend and arrange it so that she can't sleep anywhere other than underneath you regardless of conditions, eventually she'll do it without any leashes or confinement---it will become the normal thing for her to do.