If you are measuring dielectric constant in the frequency ranged from 100 Hz to 10MHz, most of the material will show decrease in the function of frequency due to space charge polarization occurred. For dielectric materials, normally we want higher dielectric constant value because we want the material can store more charges. In contrast, metal have good conductivity which may lead to lower dielectric constant value.

the mixing rule depends heavily on the porosity of your sample, particle shapes and the percentage of doping, the composition of your composite may play an additional role. Now to be more consistent, when comparing your values to literature, are you measuring the same material with the same form? you could cross compare if your values make sensing by the different mixing rule to check consistency. we did some work in the past were we looked into these parameters etc.