Ct to Viral Expression Conversion

I have an Excel spreadsheet that contains raw Cycle threshold (Ct) values for both the Citrus yellow vein clearing virus target sequence and my chosen housekeeping gene, Nad5. I need those Ct data converted into meaningful viral expression values so I can compare virus titres across my plant samples... (Budget: $30 - $250 USD, Jobs: Data Processing, Data Visualization, Excel, General Office)