Hello aleksandar95,

as mentioned earlier, using the manufacturer's specifications is a good starting point for defining the stochastic model. Values that are not specified by the manufacturer must be derived empirically by the analyst, for example based on experience, or must be set to (almost) zero. It is common practice to evaluate the stochastic model within the adjustment procedure using e.g. variance-component-estimation. A proper analysis of your data w.r.t. the selected stochastic model yields the answer to your question.

/Micha