Recently, we demonstrated that the speckle signal from a self-mixing laser diode (SM-LD) can be used for non-invasive measurements of relative blood flow in the human body. The measurement of blood flow is based on counting the number of the intensity changes in the speckle signal of an SM-LD in a predetermined time interval and calculating the mean speckle frequency. It was shown by performing measurements on healthy human volunteers that the mean speckle frequency is higher in the areas where blood flow is active (e.g. enhanced flow, along observable vessels, healthy scabs) but has smaller values on the areas where blood flow is not active or restricted (e.g. areas with non-observable vessels). The blood flow velocity obtained on those measurements is a relative value but it has consistent linear relation characteristics with the mean flow velocity. In this paper, we study the effect of background reflectance and hematocrit level of the blood on the mean speckle frequency by a simple model. On the basis of the results, we discuss the possibility of calibrating the measurements to get an idea on the absolute velocity measurements.