Need advice on designing an eight Layer PCB

I=92m designing an eight Layer PCB Board that has LVDS signals in frequency of 700Mbps. I have read lots of Document regarding Impedance Controlled PCB routing. The desired impedance is 100 Ohm as I read the device datasheet. But in some Application note I=92ve seen that the Impedance should be 75 ohm or 85 ohm. For example as I see in (part of

2.2.1) the impedance is dependent to the number of layers. What is the exact impedance that I should base my design on it? My stackup is as follow(differential impedance is 87 ohm): Top Layer: Differential signals width 9mil and 4mil for their gap Substrate: 7mil GND Plane Substrate: 10mil VCC Plane Substrate: 10mil Midlayer1: Differential signals width 5mil and 5mil for their gap Substrate: 10mil GND Plane Substrate: 10mil Midlayer2: Differential signals width 5mil and 5mil for their gap Substrate: 10mil GND Plane Substrate: 7mil Bottom: Differential signals width 9mil and 4mil for their gap

As you see the track width is differs by 4mil in top layer relative to mid-layers. How can I change the Stackup that the track width be closer to each other in different layers? Is there software that helps me or a site that has some example of stackup?( I know that the HyperLynx can do this but I don=92t have much time to learn it and I=92m looking for other ways in the little time that I have.)