Hi,
I think using the ref pose for your Additive Base could indeed be the cause of your issues. Since the additive is built from your ref pose, it contains all of the rotations and translations necessary to get your character from its T-Pose to your desired additive pose. However, when you apply those same rotations/translations to what is coming out of your BasicMotion State Machine, the result is a much more extreme pose than you intended (because you would need different values on each joint to get say from your Idle Pose to your additive pose than you would from your T-pose to your additive pose). In short, you want the additive base to be as close as possible to the animation/pose that will be playing under your additive animation. Try changing the additive base to your Idle pose (or the first frame of your Idle anim) and see if that gets things a little closer.
Another option is to set the Additive Base to be the first frame of the additive animation itself. That will basically zero out the first frame/overall pose of the additive animation and leave you with however your character animated after that initial frame.
Good luck!