Hi Seaspray,
Faced with the same questions, for my marathon, I've cut a pile o' planks about 2-5mm oversize, making sure both ends are square.
I've marked the ply subdeck on the boat with lines to keep my planking parallel and, at ninety degrees to those, lines to note the plank breaks across the beam.
I've then laid the planks in-situ, one-at-a-time, trimming off the excess on the boat before gluing them down. This means all the plank ends start and end in the right place, without the fear of having a sack of "parts" to work from of differing (or just plain wrong) lengths.
It's worked for me.
Andy