Different propagation velocity on each layer

Hi,

I need to constraint delays in terms of timing (nS) with routing across different materials in a multilayer pcb.

I use materials with 1.43 10 e+008 m/S and 1.66 10 e+008 m/S.

How can i define these different values on each layer?

I can't define constraints in terms of length, because i have to take care of package pins delays and via delays which are timing defined only.

@+