My new Tpose works well except when I import into Blender as an FBX, it's off by either 9m or .9m (depending on the import scale setting I use) along the z axis.
Edit:
Okay. I think I finally got this to work. If you import the default rig into Blender via MPFB, you have to bring the root down in pose mode a bit. For anyone else who might be encountering this problem these are the magic numbers that worked for me.
[strikethrough]Location
X: 0
Y: -0.003179 m
Z: 0.059916 m[/strikethrough]
Whew. Took two days to do this. Which is why I wanted somebody else to do it.

Edit 1 P2:
The above worked last night but not this morning. The second model I use those numbers on had the feet slightly below ground. But I
think I got a working formula now. The model needs to be imported via MPFB with Feet On Ground Unchecked. Then in Pose Mode, you grab the root bone and bring it (and thus the whole model) down till the tail of the root bone is right about level with the origin in Blender.
Edit 2:
Also, MPFB is really super duper nice. I just discovered it when working on this. This whole endeavor would have been much more grueling without it.