Thank you for reaching out. We have not extrapolated the spinal level below T12 because as you rightfully mention, there is not a linear relationship between the levels above T12 and the levels below and we didn’t want to generate false information.
Originally, the spinal level mapping was based on information from spinal levels C3 to C8 (code here), manually-identified on twenty adult subjects; more details in [Cadotte et al. AJNR 2015]. So, it is important to keep in mind that even levels below C8 are extrapolated from actual measures, and could also be wrong.