Sure. 1 and 2 are in reference to the top plane of the box I'm assuming (?). If so, I'll usually base this off of whatever visible landmarks I can find. So in this back view I found my width across the floating ribs and the depth by bringing the point of that floating rib up and to the outside of the egg. If you establish this width and depth is will be the exact same translated up to the top. I generally place the top of the box between the 7th cervical and scapula. Establishing the height and it's tilt just parallels the earlier tilts of the egg shape. Does this make sense?

Well, the width is 2 heads wide when the figure is facing front/back and 1 head width from the profile. Based on these positions I estimate the degree of turn as being somewhere in between the two. It's not a perfect science but it at least gives me some guard rails.