The Saturn PCB toolkit has been mostly suggested for similar purposes.
It should be noted that the traces on the said layers are no clean asymmetrical stripline because the impedance is affected by traces on the other signal layer. You get best results by defining orthogonal routing directions on the adjacent layers.
From this layers stack-up,if the thickness of L2 to L3 is 150um,the width of line for 50ohm impedance on L3 should be 200um.
Myabe other layers stack-up adjust a little to reach the impedance.
PS:You can use SI8000/SI9000 software to calculate it.