The perovskite ABO3 structure forms the basis for a wide range of functional and quantum materials discussed in the literature, although applications so far have been limited in part because accurate control of film stoichiometry and the ability to precisely construct heterostructures continues to lag behind their conventional semiconductor counterparts. While layer-by-layer growth mode is often cited for perovskite films, most transition-metal perovskite oxides self-assemble via an energetically-favorable layer-inversion mechanism whose phenomenology we demonstrate can be exploited to precisely adjust the stoichiometry and surface termination at any point during growth. Layer inversion results in a universal behavior of the in situ electron diffraction rocking curves and diffracted-beam intensity oscillations during alternating A- and B-site shuttered growth for a wide range of polar and nonpolar surfaces. We present a model that permits unambiguous interpretation of the oscillations, providing for the first time a completely in situ method for precise relative and absolute calibration of multielemental A- and B-site fluxes at the percent level, and apply it to successfully grow a single-phase high-entropy oxide film.