Issue with Body Orientation with Humanoid Rig

I've got a custom character that I'm trying to use some purchased animations on using a Humanoid Rig. But when I apply those animations, the body orientation is off, resulting in a slight lean to the character.

I've tried modifying rotations and positions in various different ways, (in both 3ds Max, as well as in the Humanoid Avatar config) but almost nothing seems to correct it.

My rig goes from Hip-Spine1-Spine2-Chest. Be default those map to Hip, Spine, Chest, Upper chest respectively. If I omit Spine 2, and just match My Chest to the humanoids Chest, that greatly reduces it, but doesn't fix it completely, and I don't know if not having Spine 2 mapped will come back to bite me later.

The only way I've been able to modify the lean while keeping all bones mapped, is by dramatically moving the base spine backwards, which reverses the lean. (there's a middle ground there where the spine is pushed half as far back, and there's no lean, but that's clearly not a workable solution.)

How is the body orientation determined? or what do I need to change to get things lining up right?