Design Dielectric Constant in Rogers PCB materials

Hi all,

There are two different dielctric constant for Rogers PCB material in their datasheet; "Dielectric Constant" and "Design Dielectric Constant". For example, Ro3006 has dielectric constant of 6.15 and design dielectric constant of 6.5.

I check a line calculator (MWI-2013 from Rogers itself), that uses some empirical formula to calculate line impedance . It uses dielectric constant of 6.5 for RO3006.

Here's my question: To design a line in HFSS, which is full wave simulator, should I use dielectric constant of 6.5 (the design value) or it's measured value (6.15)? I think 6.15 makes more sens.

Thanks,
Feri