What size case are you reloading? 45/70? I've used a friend's Dillon 550B to reload .44 specials with Trail Boss, and it works great. Charges are plenty consistent for my purposes (CAS, not super accurate match rounds) - if you were wondering about that. But I'm using 4.5gr charges, so I'm sure we're using the small charging bar.
I've seen some literature that suggests there's an inconsistency in Dillon's reporting which charge bar does up to what size charge - at least for the 650xl. Supposedly, pg 15 of the 650xl manual says the small bar is for up to 20gr, but pg 21 says the small bar is for up to 10gr. Also, this web page advertizing the 550B
https://www.dillonprecision.com/content/p/9/pid/23594/catid/1/RL_550B says the small bar in the 550Bis for up to 15gr. Is that what you're trying to clarify?
I looked at the 550B manual:
http://dillonhelp.com/Dillon%20Manual%20PDFs/dillon_rl_550b_manual_may_2007.pdfPage 15 says the small bar is for 3gr up to 20gr, and the large bar is for 20gr up to 45gr. I'd bet the small bar would work for your 12gr charges. But I don't have actual experience with charges that size, and Trail Boss pellets are kind of big.
Pg 13 does say to use the big bar whenever possible. I suppose you could set your machine up with the big bar, try to drop 12gr Trail Boss into some cases, and weigh the charges to see if the big bar will work or not?