In this work, we present a succinct derivation for the Square-Root model based on an explicit model from optimal control theory. Our derivation is intuitive and exact, makes fewer assumptions, and requires far fewer steps than the two-submovement derivation presented in Meyer et al. To compare the Square-Root model with statistical significance, we performed two extensive experiments with homogeneous and heterogeneous targets that appear in sequence on a plane. Later, we extend the two-parameter model to a more accurate three-parameter that increases the accuracy of the model over the two-parameter formulation. The succinct and intuitive derivation and new experimental results may enhance the appeal of the Square-Root model for design of systems and interfaces with heterogeneous targets.