The only way to get accurate impedance figure is to talk to your PCB manufacturer. The problem with pre-preg thickness is during pressing some of the resin will fill the gaps in the etching below, reducing the gap between layers. So depending on how much copper is on the adjacent layers, determines the final layer thickness (also what prepreg thickness and glass weave is used, 1080,2116, 7628 etc).
On high speed boards I get figures from the manufacturer, and feed these back into my CAD system.