A couple of interesting spectra.
With regard to choice of star for the response, as well as choosing an A or B type star the other factor is to have a good professional spectrum to produce the response curve. Stars from the MILES database are the common choice, as this has good coverage across the sky. In a previous post Robin gave a link to a very useful spreadsheet developed by Paolo Berardi which allows you to search for MILES A or B stars for coordinates of interest.
I think Castor is a reasonable choice, though Robin and David have more experience in choice of good stars for low resolution spectroscopy than I do. Though it is a double star which might make me think twice about using it, it is in the MILES database (HD060179), so that means there should be a good professional reference spectrum.