Rotating a 3D item model on the hotbar is messed up

When creating a 3D block model for an item, you can specify the rotation/translation/scale for how the model appears in the hotbar (via the gui section of the display section). While the translation and scale parameters work fine, there's something wrong with the way rotation is set up. I've been playing with this for a few days and I can't make heads or tails of what it's doing- it seems like the Y and Z axis are relative to the model, but the X axis is locked relative to the world or something. At least, the normal methods for calculating 3D transforms vis a vis rotation are yielding results that don't make any sense. Trying to get an item model to 'face the camera' and then turn it clockwise 45 degrees appears impossible.