If you are trying to patch that rear section, I would cut somewhere in between the last two cross members (ie., the last one is directly under the wall, and the next one will be about 2 ft. in front of that. So measure about 1 ft aft of the cross member, and if that takes care of all of your rot, then you are good.
The reason I say not to cut on the cross member, is that the cross members are not very wide, so you would end up with two free ends of plywood meeting on the cross member, and each teetering on about 1/2" of metal. When you put your patch in place, you will want to glue and screw a piece of plywood underneath the existing sheet so that you have a "scap" supporting your new patch and joining it to the existing sheet. All that being said, if you wanted to get fancy and cut on the cross member, and put dovetails in both the existing piece and the new piece, then they could lock together right over the cross member. Its a lot of work--probably not worth the effort.
Usually when the plywood is that rotten, the rear hold-down plate is disintegrating, and the rear-most cross member is all but eaten completely away. Have you checked out the condition of your frame back there? When I did my '73, I ended up having to replace about the last 3-4 ft. of frame rails in addition to the last two cross members.