Actually got my own answer - thank you Admiralty manual of Seamanship, vol. 1.
One deck is the first continuous deck of a ship (no ref to enclosed). So, obviously, in the St-Laurent's, the after upper deck was not on the same level as the foc'sole, because of the half deck, so "Burma Road" was the first continuous deck. On the IRO, similarly, you had the foc'sole on the same level as the flight deck, so the first continuous deck was the one running on the upper deck from the stern to under the flight deck and all the way forward to the forepeak.