I was running short of 12 mm foam so I found that if I cut all the sheets in 30 cm I could join them in one long strip and from that cut what ever lenght I need.
Another benefit from this is that it is very easy to bend and work by yourself. You can reach from a gap in the stringers and counter hold while fastening on the outside. Only the last few plank are made from 15 cm strips. I use to cut foam in 40 cm strips, but I find it harder to heat form and because shape of boat change more than on a big boat 30 cm strips work best (for me!)
Only one clamp holding piece in place. I scripe the gap to mark the cut off of the new strip. I do not use the rabbet here since I am short of 12 mm foam. Reason is that I did not understand Mr Farrier when he writes in plans that deck and cockpit floor should be made from 12 mm foam. I made the cockpit seats in 12 mm foam as well, but then I was short of 1 sheet. I am glad I made seatings in 12 mm since I will be walking on then as well as on the cockpitfloor.
I know of a canadian builder (Etienne and Annie's F82R, see link list to the right>)that made everything on the the boat from 10 mm foam.
Marking with dividers.