Check the value of for each receiver used. If the weather is reasonably good, make a skydip (OBSSKYDIP command) to measure the forward efficiency. In case of scattered clouds, you may prefer to rely on the preceding measurements of this parameter. In any case, the knowledge of the image band gain ratio, , is essential. This is mostly a scale factor when signal and image opacities are similar, but this is not entirely true close to strong atmospheric absorption bands (e.g. at 115 GHz).
The observing strategy then depends on the weather and (or) receiver stability. Under good conditions (stable weather), you may calibrate with a CAL COLD from time to time, and use the derived value of the water vapor content for intermediate calibrations with a simple CAL AUTO. If the weather is not stable, but the receiver is, use a CAL TREC instead of the CAL AUTO. CAL COLD is always best since you can monitor both water vapor and receiver fluctuations.
Do not forget to focus (OBSFOCUS command) the telescope for the receiver you are most interested in. Otherwise, the standard value of the beam efficiency will be wrong.