6.17.7 references Table 6.6 which refers to T, nominal base metal thickness.  Table 6.6 DOES note that T, nominal base metal thickness, may be increased to provide for the thickness of allowable weld reinforcement, provided shims are used under hole IQIs (per 6.17.3.3)

6.17.7 also addresses the number of IQIs (for complete circumferential pipe weld) and says that steel backing shall NOT be considered part of the weld or weld reinforcement in IQI selection.

I believe that the answer to your question is found in 6.17.3.3 REINFORCEMENT. 

Good luck