Im not that big fan of the lvl 6 nissan, i preffer the lamborghni gallardo when it comes to best time in lvl 6 1/4 mile races. But im gonna help you of course! Here is what i use on my no so played Nissan GTR.
Launch and 4400 - 4500RPM 1st to 2nd gear at 7000RPM + you activate the NOS at the same time you hit the clutch. 2nd to 3rd gear at 7500 - 7600RPM. 3rd to 4th gear at 7400 RPM. Then you let it over rev till the finish line. You can get around 8,896 every time you do this correctly.