How does SAM calculate the volume of molten salt in Power Tower Molten Salt ?

Hi, 
I am trying to manually verify the calculations of the Power Tower Molten Salt model but I failed to figure out how the available HTF volume. 
I referred to the "Technical Manual for the SAM Physical Trough Model" by Michael J. Wagner and Paul Gilman. I found equation 4.2 that calculates the volume of the HTF (See page 81). Is the same equation applicable in the power tower with molten salt and? If so, how to control the value of the heat exchanger de-rate factor (fhx) in the model? If not, would you please guide me how the volume is calculated? 

Thank you so much for your time and consideration 

Regards,
Abdul Hamed 

Many thanks Paul,

Would you please provide me with the equations that SAM uses to calculate the specific heat and density of the molten salt. The equations that I do use give different values . Those are the equations that I do use:

Density=2090-0.636* Average temperature ;      % (Kg/m3)Density oh hot HTF
Specific Heat=1443-0.172 *Average temperature;     % (J/Kg.K) Specific heat of hot HTF

Best Regards,
Abdul Hamed 

Thank you so much Paul.