impedance control with different prepreg styles in a dielectric

Hello guys.

My layer stackup looks like the picture

Lets say that:
- Dk for 2116 is 3.4
- Dk for 7628 is 3.7

I need a 50 ohm impedance in layer 1 (layer 2 is ground).
For impedance calculations, how do I manage two different values for Dk?

I had a look at the document below but it doesn"t help too much
http://www.isola-group.com/wp-conten...es120508-1.pdf

Thanks!

For an exact calculation, you need to model the microstrip line geometry with layered substrate in an EM (respectively AC electrostatic) solver. For an estimation, a weighted average should work. Considering fabrication tolerances due to substrate thickness, resin composition and etch width variations, don't expect better accuracy than +/- 10 % impedance.

Impedance control has to be performed by the PCB manufacturer, you are doing at best designed impedance.

So...
Should I rely on them to get the pcb's built without requesting layer stackup details from them previously?
What is the common practice?

Regards