The conductivity of a scale or deposit on a tube can be estimated by using a volume fraction average of the thermal conductivities of the components assumed to be in the scale.  A porous scale may be
composed primarily of the fluid on/in the tubes, whereas a non-porous hard deposit may have little of the process fluid in it.  I caution you that a fouling factor which has been presented on a heat exchanger specification sheet may well be an arbitrary value which someone has assigned based on common practice or TEMA tables or internal company practice.  This may or may not be representative of a specific fouling material/thickness and the heat exchanger designer should be consulted.