I did this using a simple linear interpolation for the 30- and 40-degree values referenced in the table contained in the link.
Does this seem like a sound way to determine the appropriate z-factor? If not, could anyone suggest an alternative?
No, it's not a linear relationship, but it's not that much of an issue as long as the z factor is roughly correct.To calculate it properly:z=1/(111320*COS(latitude*pi/180))
z=0.00001148 where latitude = 38.5
In Excel you can use the following formulas:=1/(111320*COS(38.5*PI()/180))
or=1/(111320*COS(RADIANS(38.5)))
See this page for further info.