How do I determine the correct tile trim depth?
I would experiment a bit using some cutoffs and a steel combination ruler.  Try to figure the average float percentage, I would expect 10-30% of the notch height for tiles that large.

The depth of mud under tile is variable, mostly based on the wall flatness and somewhat based on tile size.  You have to float enough to bridge and give full support, more so with a natural stone tile like travertine.  

Your edge profile will look best and protect best if its as flat to the wall as possible.