The process of reactive sputtering from a silicon target in an Ar/O-2 gas mixture is investigated. An optical plasma regulating circuit using the intensity of the 251.9 nm-Si I line as setpoint is employed to stabilize the sputtering process. A phenomenological model of the process is derived that, in contrast to previous models, it allows to calculate the composition of the growing SiOx film exclusively from measurable process parameters and accounts for the non-uniform deposition at the substrate. Film compositions x, predicted this way for different setpoints were verified by Rutherford backscattering spectroscopy. As an example, a SiO2/SiO1.5/SiO2 layer stack was prepared and via anneal transformed into a thin layer of nanocrystalline silicon embedded in an oxide matrix, a structure with potential for application in memory devices and Si-based light emitters.