How do I know which fibres to specify in the "--fiber-splist" command line option for the gimasterflat recipe?

The information is available from the FLAMES/GIRAFFE QC web pages. In a web browser navigate to:
 http://www.eso.org/observing/dfo/quality/GIRAFFE/reports/FIBRE/CURRENT/Medusa1/trend_report_Medusa1_CURRENT.html  

Select the relevant mode and plate from the list of groups near the top of the page [Medusa1, Medusa2, IFU1, IFU2 or Argus].  In case your data are from prior to the current 3 months you will need to look at the historical plots. Click on the "history..." link (which will open a new window) and navigate to the three month period including the relevant date.  Then, in the "Information and research:" section below the graphics, select the "measurements" link. This should display a table of information listing (amongst other information) the number of fibres (N_fib, 2nd to last column) and missing fibres (miss_fibres, last column) information for each file included as a data point in the plots.  You will then need to specify the list of fibres excluding the missing ones for the --fibre-splist option.  For example, for processing IFU2 data from 2010-12-12, first navigate to:
  http://www.eso.org/observing/dfo/quality/GIRAFFE/reports/FIBRE/2010/IFU2/trend_report_IFU2_2010-10.html  

Then click the "measurements" link, which displays the table:

The 10th and 11th entries (listed in reverse order in the table) bracket the desired date, and both list FPS192,303 as missing. The appropriate esorex command line option to specify is thus: --fibre-splist=1-191,193-302,304-315