Not sure what you mean by
Obviously there is the big ass allen/hex screw at the bottom
There are three - the lower one is the one that the rubber of the saddlebag/pannier seats against. It has a 6mm thread.
The one near to the silver frame at the front is a 5mm thread, and the one that secures the side panel to the rear cowl is also a 5mm. That screws into a thread on a metal clip.
'Big Ass screw' sounds more like the one that is used on the 1100 - except that is a cross head socket.
In my experience 4 out of 4 of those top edge rear joints have failed at the plastic - and they are the only ones that have failed on my bikes. Not as a result of wrong screws, or overtightening. They just seem to break. I tried to repair, but both of my bikes have spent the vast majority of their life not connected together at all. Hidden by the bottom of the pillion seat. In fact only the other day I found nut, metal clip and broken red plastic in my parts tin.
Never tighten these bolts much - they seem to be tighter when you take them off, than they are when you put them on. Just until they meet resistance, and a tiny nip more.
https://www.st-owners.com/forums/threads/st1300-maintenance-fairing-installation.156536/
The above link (download the pdf) tells you what screws / fastenings go where , shows you what they look like and gives the part numbers - although it doesn't mention the side panel.