fourth or fifth, and both of them are the corner: room 4 burns 9.7 a second, room 5 burns 10.8, and every room after the first caps your entry fuel at 72. so that is 7.4 seconds in room 4 and 6.7 in room 5 to find six or seven torches. room 4 is where the entry cap first bites, which is probably why that is where the thumb starts losing.
noted, and thanks. it is the only room number I have from anyone but me.