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PCI Trace Velocity

Hello PCI experts,
The PCI specifications ver. 2.1 section talk about the PCI trace impedance and velocity. It says " The trace velocity must be between 150ps/inch and 190 ps/inch".
Talking to several PCB manufacturers, I understand that the parameter which effect the trance velocity is the material between layers, and the most popular is the epoxy glass.
The equation for the trance velocity is:
T (ns/feet) = 1.017 * square_root(0.457 * Er + 0.67)
Er - The material dielectric constant.
For epoxy glass:  4.5 <= Er <= 4.9
Even if we are taking the average value of Er = 4.7, then we have T= 1.7 ns/feet --> 142 ps/inch. i.e. we are under the specifications limit.
I will appreciate if somebody who faced this problem before can share his experience concerning this issue.
Do I have to use some other material instead the epoxy glass ? Or maybe there is a mistake in my calculations ?

Yoel Lavian