To transform the spectrometer channel width from Hz to km/s use:

                     d(nu)/d(velo) = nu_rest/c       (1)

Example: water masers.

nu_rest = 22235.0798 MHz; c = 2.99792456e+05 km/s
At Medicina we usually have channels of width d(nu) = 9.7595 kHz.
Using the equation (1) we find this corresponds to d(velo) = 0.132 km/s.

To calculate the sky frequency of a line:

First calculate the Vlsr of the observatory (Vobs), using
http://fuse.pha.jhu.edu/support/tools/vlsr.html

Input: date and time (UT) of observation, longitude, latitude, and elevation
of observatory, and the coordinates (RA, DEC, and their equinox) of the source.

Then, if the Vlsr of the source is Vsource, the sky frequency (nu_sky) is:

nu_sky = nu_rest - (nu_rest/c) * (Vsource + Vobs)     (2)