dgrenon
Firstly, what the heck is a TJI?
When you measure from the finished wall to the center of the toilet flange (12") you will be on the edge of a joist, is that right?
You could use a offset flange or you could purchase watercloset(s) (toilets) with a 10" rough-in from the wall to the center of the flange. Briggs has one and Caroma made Australia is another brand that comes to mind. Perhaps there are other ones.
Hope this helps. John Boy