A histogram is pretty useless for judging exposure it there is nothing big and white in the shot. A better tool for judging exposure is the Over Exposure Warning and a white terry towel.
Adjust exposure until clipping occurs in the towel, then reduce exposure by 1/3 stop (one click). The clipping warning is based on the JPG display image, but you can corollate the point where you see clipping on the camera OEW with when a loss of detail begins to occur in the RAW file.
Indoors M is always better on the camera for consistent control. To show the lights in the tree you\'d want to use a tripod, lower the ambient room lights and drag the shutter to about 1/30th sec. at f/5.6 or whatever aperture you select until the lights show OK in an ambient-only shot. The room will be dark. The turn on the flash in ETTL mode. Using the crumpled towel next to the face as a guide adjust exposure via FEC until the towel is 1/3 stop (one click) below clipping. I normally wind up with FEC around +1-1/3 with the diffusers.
