I've updated WebSpec with Suzaku AO4 files. The HXD background files supplied had a total exposure of 3e6 seconds. The recommended procedure is to have a background file with the same exposure as the source file. This assumes that statistical errors on the background are Poisson and the systematic error can be represented by changing the total normalization (using corfile). I modified the standard background files to change them from COUNTS to RATE so an input background file could be created for a given exposure time just by changing the EXPOSURE keyword.
Note that fakeit with a background file creates a simulated source spectrum with the exposure requested and a simulated background spectrum with the same exposure as that of the input background file. A useful enhancement for fakeit would be to control the exposure of the simulated background file - one possibility would be to allow commands such as
XSPEC12> fakeit inbackground.pha 50000
UPDATE 11/04/08: Fixed an error on the energy ranges for the PIN. The input background spectrum is only valid up to 75 keV. Also, use an background exposure time of 10 x that of the source for the PIN as recommended by the HXD team.