RF microstrip track parameters calculation
I can recommend MTTL (sourceforge page) which could handle this task. It is a 2D field solver and therefore can cope with many non-standard geometries where the simple empirical calculators cannot deliver an answer.
As far as I remember it should give you all parameters required for a CPWG. You could simply play with the parameters until you get the desired impedances. Of course you can use the results of simple empiric calculators as a starting point.

There are also many RF packages out there which could give you the same answers (with a polished interface), but they tend to charge you a lot for them.

Edit: Here is a step by step guide for MTTL (TNT):


choose your unit system (top right), e.g. microns
add a ground plane (New Ground Plane button) and specify the parameters (e.g. 35µm)
add a dielectric (usually FR4, but use whatever you plan to use for your PCB) e.g. 1500µm FR404
add your signal layer ground structure by first adding rectangle GND conductors (button New Rectangle Conductors) e.g. enter as 


Name GroundSignal, 
Conductivity: Copper, 
height: 35µm, 
width 10000µm, 
number:2 
Pitch 15500µm.

add your signal conductor: add rectangle conductor with values 


Name:Signal, 
Conductivity:Copper, 
height: 35µm, 
width: 1500µm, 
x offset: 11500µm = 10mm + (5.5mm - 2.5mm)/2 = (left gnd width + (gnd pitch - signal width)/2)

Save your file.
Run the simulation: BEM> Run> Run
Click show results: Find the line 

Characteristic Impedance (Ohms):
For Signal Line ::SignalR2= 53.0741
From there adjust the values until you get the desired results.


The results closely match those of the simple calculators (as mentioned by Alper91), but you will be able to also account for special cases not covered by them (e.g. slight asymmetries).