Transient analysis of square wave using PSPICE
What I do: 

128 kHz = 7.8125us so your PER is correct.

Then I make PW = PER/2 but subtract half of the the total of the transition times:
TR + TF = 20 ns => half of that = 10 ns.

So PW = 3.90625 us - 10 ns = 3.89625 us.

So make PW = 3.89625u and then you would have 128 kHz with a perfect 50% duty cycle.