As it happens – Francois Teyssier has just updated his reference star finder for low resolution spectroscopy
http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=1227
It finds the potential reference stars nearest in altitude to your target and now will even plot a trajectory so you can see how the relative altitude changes with time
It now includes all the MILES A and B stars (which have professionally measured spectra) as well as a long list of bright main sequence A and B stars with low interstellar extinction and published spectral classification. These should have spectra similar the generic (eg Pickles) spectra for the particular spectral type but they do not have published spectra so you need to be a little careful using these, perhaps at some point comparing them with a MILES standard for example to confirm they are typical of their type.
I think measuring a few MILES stars of different spectral types is good practice if you are starting out or testing new equipment as any deviations between the measurement and the database version can be seen and followed up to improve one’s technique. There are some examples here where I did this using the Star Analyser and ALPY
http://www.threehillsobservatory.co.uk/astro/spectroscopy_21.htm
I find getting the blue end below ~4000A perfect particularly difficult for a number of reasons:-
the sensitivity of the instrument drops off sharply
any errors due to atmospheric extinction are greater
any problems from selective sampling of particular wavelengths by the slit due to for example atmospheric dispersion or chromatic aberrations in the optics are greater
it is difficult getting enough signal from the (halogen) flat lamp to keep the noise level down
the increased crowding of the Balmer lines towards the Balmer jump makes it more difficult to get a perfect match between measured and reference spectra.
Cheers
Robin