I have done just that. I have a pretty considerable difference between BPEL (8") and strictly NBPEL (7"). My "insertable" length is just between (7.5").
In those individuals with minimal fat pad thickness in whom BPEL and NBPEL are fairly close to each other, I suspect that the insertable length will be close to NBPEL. There would be minimal subcutaneous tissue to displace, just skin, a very thin layer of subcutaneous tissue, and ligamentous connective tissue, not very compressible or displaceable.
In those with thicker fat pads the insertable length might be closer to the BPEL than the NBPEL.