What I used sometimes is the Er which is in the middle between the two Ers or I use the Er value of the thinner dielectricum which is placed closer to the reference plane. But the value is always not so right, so it is better to ask your PCB manufacturer.

What you need is one of the reasons why it is important to ask PCB manufacturer to provide the final stackup and track geometry. They can calculate the values more precisely.

I have learned a lot from your article. Quite useful and impressive info.