The antenna gains (in K/Jy) are simply the ratios of the measured visibilities of a point source (in units of antenna temperature, as obtained after real-time calibration) to the flux of the source. Unfortunately, most phase calibrators are time variable sources, and have frequency dependent fluxes. The IRAM staff regularly monitors the fluxes of the bandpass calibrators at 2 frequencies. The flux of your phase calibrators at your observing frequency is not generally known at the time of observations. It must be derived by bootstrapping from measurements of sources of known fluxes.
To properly bootstrap the fluxes, measurements should be as close in time as possible (to ensure reasonably equal phase noises), and preferrably pointing scans (POIN) should be used to correct for possible pointing errors. Of course, the focus should not have changed between the measurements, and atmospheric calibration should be good. In this case, the antenna gains are identical for the flux calibrator and the source(s) whose fluxes must be measured.
Sources of known flux are specified using the SET FLUX Source Value command. After the data have been selected, the SOLVE FLUX command calculates the antenna gains. Then the STORE FLUX command stores the fluxes for all selected scans.
More details about flux measurement are available in the ``Flux Measurement'' documentation. Bad determinations of the flux scales can create important artefacts on the final images. This determination is particularly tricky: consult IRAM staff.