Polar Imp Calculator Doubt

[shivas]

Dear All,

I Have some problem in calculating the impedance using polar.

Here is the layer stack-up:

Layer 1 - Top (GND)
prepreg
Layer 2 - SIG 1
core
Layer 3 - GND Plane
prepreg
Layer 4 - SIG 2
core
Layer 5 - SIG 3
prepreg
Layer 6 - PWR Plane
core
Layer 7 - SIG 4
prepreg
Layer 8 - Bottom (GND)

I have pour the top/bottom layers completely with GND and have only fanout.

Now I need to calculate the impedance using Polar with correct field solver which I don't know how to select, for example to calculate the impedance of Sig2 and Sig3, I have two models namely

1) Offset stripline 2b1A and 2) offset stripline 1B2A. I need to know which one to use and what's the difference between two?

Need help soon. kindly post some presentation to look into the model stackup.

Thanks
Shiva.