Dave achieves this by tilting his head left (port) then right (starboard) with suitable chanting !
He then does the same along the baton below, marking it on to the ply.
This shows the space from these drawn lines to the edge of the new plank.
Then he slides the block along the lines he drew on the ply before and carefully draws along the opposite side of the block.
This gives an exact copy of the space the plank will fit in !
This is then put through the planer and a small angle added to the top and bottom of the plank to allow for caulking later.
This is then clamped tight against the frames (ribs) and also the plank above .Pilot holes are drilled in the plank to the frames and nails hammered home for a tight fit.
( This gives Dave even more to do !)
I now use this technique for making templates around the house.My wife is amazed and now I have more work to do !!!!!!!!