I`m pretty sure that it`s documented, but if you think about it, the FC shouldn`t really need trim after transmitter calibration. It applies the required thrust to maintain either the GPS position, atttitude, or rate depending on what it is sensing, the flight control input, and flight mode selected. In the case of the NAZA, at 50% throtlle, it will attempt to maintain the given altitiude in GPS mode. I`ve observed this in the 4 different NAZA equipepped MRs that I own. If it is not doing this, then IMO there are probably non standard factors in play, such as an underpowered or misconfigured aircraft, wind speeds or density altitude outside of the demonstrated flight envelope, or a battery with bad cells or low charge.
It`s pretty easy to observe with an OSD which displays verticle velocity. For me while flying FPV, having the throttle beep at the midpoint is really helpful in ballparking 50%, and and then the OSD backs up what I am hearing.