Please can someone provide me the conversion formula from ascii to mg/m**3 of Chlorophyll a concentration (SeaWifs)?
Thank you!

For what product? For chlorophyll, Level-2 and Level-3 bin products are already converted (stored as float).  Level-3 maps (and, generally speaking, all of our HDF formats) have the required scaling information embedded in the HDF SDS meta-data.  This is explained in the format specifications:

  http://oceancolor.gsfc.nasa.gov/DOCS/ocformats.html

In general, it is a bad idea to assume there is one static conversion.  The scaling may change between reprocessings and even within a reprocessing version.  It should be read from the file, for every file.  This is what seadas is doing.  If other methods are used to read and analyze the data (matlab, envi, basic idl), those methods should be developed with similar sophistication.

-- bryan