Systems and methods for determining the elbow or Ct value in a real-time, or kinetic, PCR amplification curve data set. The PCR data set may be visualized in a two-dimensional plot of fluorescence intensity vs. cycle number. A rotation transform is applied to the data set to rotate the data about a defined coordinate such as the origin so that the data point representing the Ct value becomes a minimum or a maximum along the intensity axis. The data point representing the elbow or Ct value of the PCR curve is identified, and this data point is then rotated back and the cycle number of the data point is returned or displayed.