It looks like the roof was shipped in three peices and assembled on site. I haven't seen this setup before, but I think there is nothing to worry about.
There are four measurements that you are going to need. Accurately measure the 27 ft side of the house including the siding. An accurate measure of the angle of the roof. Set a 4 ft level on the roof hold it level and measure down to the roof and divide by 4.
The third one is the height of the seat cut, the distance between the top of the wall and the bottom of the sheeting, measuring straight up from the wall. Hold a level against the wall and draw a line on the side of a truss and then measure from wall to bottom of that line.
The fourth is the tale length the distance between the bottom of that line and bottom of the angle cut at the end of the truss behind the gutter.
We need 1 and 2 now and we can get 3 and 4 later.
Will you be ok with removing some drywall to add structure to the wall?