How to calculate virus titre from qPCR
I have 2 doubts here. 


You obtain your viruses from the media whereas Actb measurements can be performed only in the cell lysates. 
Actb is just a reference to normalize for the cell count: higher the cell count higher will be the viral titre because of higher number of infected cells. But that is immaterial if your aim is lentiviral transduction where you are only interested in obtaining titre values and not measuring the efficiency of its formation. 


You can simply do qPCR with the media sample and the standards and regress the measured Ct values of the samples to obtain titres.