"With the inner not attached, how do you "set" the length?'
Not sure if I have a video showing that but it is rather simple.
Set up the tent with the inner attached.
make sure it is set up correctly.
Now remove the inner and the the trekking poles but leave the two end set up.Take a look at how the fly collapses.
You will notice that in the middle the fabric has some slack .
So knowing this ,take it apart and set one end up, go to the other end , set the other end triangle to be parallel to the first one and leave some slack on the fly for the middle.