In Eq. 3, is the antenna to source coupling factor, required to obtain a scale which corresponds to the brightness temperature of your source (which I presume is the only physical parameter which really interests you).
If you knew your source structure exactly, this factor could in principle be computed from the antenna radiation pattern. If the source is unresolved, it is customary to use the so-called ``Main Beam Brightness'' temperature scale, which would be the brightness temperature of a source just filling the main primary beam, hence using = , the Beam efficiency. If the source extent is unknown, you would use = , the Forward efficiency, pending further correction ( scale, with no direct relation with the brightness temperature of the source).
At the 30-m, is called B_Eff in OBS, and it can be set using the OBSSET EFFICIENCY B_Eff F_Eff command. ( is also noted F_Eff in OBS manual). is only a scaling factor in your temperature scale.
At Plateau de Bure, is assumed equal to F_eff, because a secondary amplitude calibration is required.