At the risk of complicating what has so far been a fairly simple issue, there is another way of approaching the task of shortening a kayak.

I lengthened my wife's 4.5 metre kayak by about 10% to produce a 5 metre version for myself. However instead of just increasing the gap between all forms by 10%, I didn't change the stems at all. I added a few millimetres to the next gap, a few more mm to the next gap and quite a few between the forms near the middle.

The effect of this was to keep the shape of the bow and stern very much the same, but because most of the increase in length was in the middle where the kayak is wider, the increase in volume was more than 10%.

If we take all that in reverse, then if it is important to you to maintain as much of the volume as possible then the ends could be progressively shortened more than the middle. If on the other hand you want to reduce the volume so that with a much lighter paddler, the kayak sits in the water where it was designed to do, you could reduce the middle more and have progressively less change towards the ends.

Or you could keep it simple and just change everything by 10%. It depends a bit on what you are trying to achieve and on the weight of the paddler using the kayak compared with the weight it was originally designed for.