Hi Twelvv,
When you press the Set T button you are simply setting all the joint
rotations to zero.
Therefore, it's very likely that you did not set the reference
position correctly when recording the data.
Theoretically, you could manually move the bones into a t-stance and
save this single frame as your reference position, but how you would
use this depends on how you intend to use your data.
Other members may offer other ideas...
Good luck!
- Davedub