Adapting an IES file for different luminaire configurations (e.g. using an IES file for a 4' linear luminaire to run calculations for the 8' version of the same luminaires) can be done by making a few adjustments in Luminaire Manager.
Step 1- Adjust the Luminous Dimensions to the appropriate values.
Step 2- Prorate the luminaire output. The Proration Factor acts as a scalar multiplier on the candela values in the IES file (similar to Light Loss Factor). For the example of scaling a 4' IES file to 8', a proration factor of 2 would likely be appropriate. For the correct proration factor it is best to consult the manufacturer's documentation (cut sheet) or contact the manufacturer directly.
Both the Proration Factor and the Luminous Dimensions are stored as Shared Parameters on the luminaire family. If desired, use these to parametrically control the proration and luminous shape using formulas like the ones below. Note the ElumTools Shared Parameter file is located at C:\ProgramData\Lighting Analysts\ElumTools\SharedParameters\
An example of a parametric linear suspended family is attached below, for reference.