This may be an overly simplified concept from an overly simplified mind when it comes to tailoring, but what about making a "proof of concept" when she is available for measurements. Measure what she says is the length she wants, cut a piece of cheap fabric at that length and have her pin it on to see how it works. That would let you, and her see in real life how that length would work without sacrificing expensive fabric or loads of sewing time. This may not help with the concept of needing to taper the back for modesty's sake but at least you would have the jumping off point.