is how far forward she sits in her boat.  if it is somewhat forward, then the tunnel (the part that actually fits around the torso/waist) may actually need to be sewn an inch or two forward on the skirt, otherwise it will cause her to hunch over and/or stretch the skirt.  make sense?

never thought about that, but i was lucky enough to have John Mason of Mountain Surf (GREAT SKIRTS, GREAT FOLK!!) evaluate where i was sitting in the boat i just bought, and then they custom made my skirt.  it has made all the difference!  (thanks john!)  you may want to give them a shout.  their website is http://www.mountainsurf.com

hope this helps! 

peace & love, Charly