They want to you increase the EGC by the same ratio you increase the current conductors. These #s are availible in Table 8
For example, lets say you have a 125A circuit (Normally #2 Cu w/#8 EGC) but have to increase the conductors to 1/0 to allow for derating:
#2 = 66kcmil
1/0 = 106kcmil
Increase is 60%.
#8 = 16.5kcmil.
16.5kcmil * 160% = 26.4kcmil
Looking through the table, #6 = 26.2kcmil, and would be the proper ground wire to use.