Distributed voltage control using Q(U)-characteristics is a means to facilitate higher penetrations of decentralized generation. However, stability concerns interfere the wider adoption of this technology. Based on a recent introduction of a robust stability criterion for assessment of droop-controlled inverters, this paper aims to improve the modeling of the grid from a control engineering point of view. After analyzing the impact of different types of voltage deviation on nodal voltage sensitivity, a more precise grid model is presented using algorithmic differentiation. The derived model is further implemented in a closed loop scheme for a Q(U)-controller. Considering in a first step of evolution solely a single input single output system the existing stability criterion could be improved in terms of accuracy.