The tumblehome panel needs to go all the way to the stem to make a point. If you did this on your current boat it would be too low at the ends because your sides are the same height at the stems as mid ship. Had you cut the side arc out of a board or panel the stem height would be greater than midship. You formed a panel out of strips with parallel sides. I shaped the sides on my "Truck Boat" build like that and the TH panel did not come to a point. Resembled the shape of yours. https://www.southernpaddler.com/community/threads/truck-boat.9289/
One function of the TH panel is to raise the height of the sides at mid ship.
If the depth of your boat is 10" with the TH in place the ends should be that or a little more depending on your taste. Hope this will help:
The "Plank " part may not be technically correct. More like "solid wood panel". I did not have any suitable planks so I am making/using shorter boards jointed and glued into panels. For sometime I have been buying any "clear/solid" cedar fence boards I ran across. There is some waste and...www.southernpaddler.com