When I am on thief with max th+ gear, I always wait until the initial "level up" message in the log appears before switching to my DD set. If I claim the mob with max th+ then switch before receiving the message, it will start at the base of 4, and increment one at a time thereafter.
There is one notable exception, which SE did note in recent-ish patch updates, which is TH now has a chance to proc/level up during a weapon skill. So if that is the case, sometimes the first message regarding TH I see is that it moved to 9 (or in the case of treasure hunter being in affect, 10).
I like turtles c: