A grain transfer control system for automatically controlling relative positions of an unloading system discharge nozzle of a work machine and a receiving container based on a real time model of a fill level profile for regions of the receiving container, the profile being modeled using known or estimated rates of flow of grain and positions of the nozzle relative to regions of the receiving container as a function of time, the system adjusting the relative positions of the nozzle and the receiving container to effect a generally even fill of the receiving container.