Digging way back in the brain cells -
Boeing always called for 15% N2(/N3) for the 747, because that's what the JT9D people required. The CF6 started very nicely at 10%, which was a good thing because the APU in the DC-10 had to be derated to prevent bad things in that department. Often DC-10 max motoring was no more than 11-12%.
But Boeing still called for 15% for standardization between JT9D/RB211/CF6 donks.
