The Ct (Cycle threshold) – or more correctly – Cq (Cycle quantification) – value is per definitionem being used to determine the gene expression via the fluorescence. Basically, one determines mathematically (commonly used is the 2nd derivative-method) the value where the fluorescence does not increase any more which indicates that the exponential phase of the amplification has ended. This happens in a certain cycle which is then called the “quantification cycle” (Cq).
However, there are many more mathematical methods to determine the Cq-value (see literature below). Most important is that you always use the same calculation method within one experiment.