If you screw it up, you might as well throw out the A/C system. (Good disclaimer; it could be botched easily.)

There isn't really a trick to this. It is based on the low side pressure of the system. That means that the R12 system usually cycles the compressor off at around 25lbs, but with the R134a system, you can effectively reduce the cycling pressure to around 19-21 pounds without icing the evaporator.

The temperature output of the system is directly related to the pressure of the low side. You can theoretically reduce the pressure so much that the system cranks out ice-cold air (like zero degrees) but unfortunately, it will also cause massive icing of the evaporator. Therefore, it will not be possible to operate the system over a period of time; you will have to reduce the system settings or turn off the system before it will work properly after it ices.

If you (or someone) are doing a conversion, please follow the low side pressure gauge to adjust this switch. Failure to do so will almost positively result in a lack of output capacity due to icing, or worse.

This is a very good tip. To properly convert a clutch-cycling system such as the Volvo 700/900 to R134a, that switch should be adjusted. Not doing so will reduce the ultimate success of the conversion. It won't harm the system, but it will be lacking some of the capacity it could have with a slightly lowered cycling pressure.
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chris herbst, near chicago