Re: impedance control with different prepreg styles in a dielectric
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.