Step 8: Transfer body measurments to the top edge of the gunwales
Make a mark 17 inches back of the centerline you marked earlier. This will be the location of the front edge of your back rest. Make another mark 1-1/2 inches back of that. Your back will be 15 inches back of the boat's longitudinal center. The extra two inches will give you enough room to add some padding to the back brace or to install a back band to support your back while you're paddling.
Take your back to kneebrace measurement and add 2 inches to it. Measure from the backbrace front edge forward by this amount. This will be the front edge of your knee brace. Make another mark 1-1/2 inches back of that and label it knee brace.
Take the measurement from your back to the balls of your feet, add 2 inches and measure forward by that amount from the back brace mark. Make another mark 1-1/2 inches forward of that. Label this foot brace.