How to describe this – here goes,
In plan you have the centreline or keel, radiating from this are several frames, each from the bow get further and further from the centreline as they get further from the bow, they them start to come back to the centreline as they get closer to the stern, so far so good.
Each frame marks the furthest the hull gets from the centreline and generally is curved and intersects the end of each frame giving the profile of the hull, this curve means that taking different frames there will be a difference in how far the frame is from the centreline.
Frame 1 near the bow is say 100mm from the centreline at deck level, it is also concave to the keel to give the usual knife cutting profile.
Frame 2 is 200mm further back and 200mm away from the centreline at deck level, it is also convex, going flat at the bottom before reaching the keel, bucket profile.
Intersecting the ends on the plan is a curve, now, say you wanted to put a stiffening frame or as I call them, splitters, half way between frame 1 and 2 this would be positioned 100mm after frame 1, it would also have a profile some where between frame 1 and 2, but not a half way version, this is because when you look at the plan, the curve at half way between the frames may not be 150mm from the centreline it could theoretically be 170mm, but the closer you move towards frame 1 the quicker it reduces to near the 100mm mark.
This means that to place the stiffening frame at half way the true distance (ie say the 170mm), will be a percentage of the distance between frame 1 and 2 where 200mm will be 100% and 100mm will be 0%, 170mm would be 70% ( 200 less 100 = 100 divide this by 100% means that for ever 1mm past the frame 1 length will equate to 1%.
So the stiffener will in theory be 70% of the difference between frame 1 and frame 2 so that by the time the morphing gets to frame 2 from frame 1 it will have changed from concave to convex.