To measure the distance between your footrest and seat, sit next to the boat on the ground in roughly the right position and adjust the footrest/seat accordingly.

Sitting in the boat (ideally on the water to reduce the chances of damaging the boat – oh all right do it on a pallet!) fine tune the footrest to seat distance. The balls of your feet should be against the foot rest with your heels fixed slightly behind. You should be sitting upright, with your hips rotated forwards so that you are sitting on the front part of your sit bones. You should have your legs moderately bent so that when you push with one leg and almost, but not quite, straighten it (with your heels fixed in place and not moving), your hip should swivel fully in the seat (it shouldn’t push your bum over the back of the seat). If you push with both feet, you will be pushed over the back of the seat.

You shouldn’t have the footrest-seat distance so close, that your knees are very high as that can cause you to hunch up and bend your back. If you can’t almost straighten one leg at a time, your footrest is too close to the seat.

It can take a bit of trial and error to get it right, but when you do, it’s worth measuring the distance from the centre of the footrest to the lowest part of the seat. You could cut a bit of plastic piping to the right length as a quick measure or simply use your paddle as a measure with a bit of tape to show the right length. That way when you use a boat that someone else has been in, you can quickly adjust and go.