My USN58 is set with the reference level at 64dB set at 50% FSH, and I scan according to the chart at the bottom of the page of Table 6.2 which is dependant on the sound path length.....through 2.5 inches = 14db above my 64db ref.
My older USN50 had a ref level of 47dB at 50% FSH.....
So to answer your question, yes, I have my newer machine set a bit higher than your TPI.
Maybe get your UT guy and the TPI to set up(calibrate) off the same IIW block, I mean check both verify the BIP, verify the Angle, verify the Distance, and verify the Sensitivity and then see where each machine ends up....note the differences and go inspect the joint again.
edit: Someone else mentioned that he might be calling the backing bar...maybe...so use the sound path and some trig(just bypass the trig in the machine for this situation)...map it out and see where the sound is going. I use ACAD when in doubt, but graph paper will work as long as you keep everything to scale.