I understand your dilemma. I just finished a quilt for a woman who gave me some fabric as a start that I had no idea how I'd make them work together. When I did the mock up, the lady saw the same thing I saw. I also did a mock up w/ what I envisioned and that helped her see the difference.
I think some people can't really visualize the end product and you have to help them see that. Many can't even visualize the stack of fabrics being transformed into the end quilt.
Take pictures of the design board as you put things together to get her approval. Couple it w/ what your vision is and see where she goes. If she sees the projected end quilt, likes it, approves it, then you've done your due diligence and there shouldn't be any surprises.
good luck!