A/c Low Pressure Switch

Originally, the spec for the low pressure system was 25 psi. This is the cutoff, so if the system pressure on the low side drops below 25psi, the compressor will cut out until the pressure on the low side rises to about 45psi

You charge with 80-85% of the capacity of R12 with the R134a as you cannot garantee total removal of all the R12's oil, so you run the risk over overcharging if you try to fill with 42 ounces of R134a. The behavior of the R134a will then typically cause it to run slightly higher pressure on the high side, and slightly lower on the low side. In other words, there is a larger pressure drop across the orifice. Since the pressure will run lower on the low side, you want to drop the low side pressure switch down to 18-20psi or so.

Setting it at 28psi will cause the compressor to cut off prematurely.

What pressure readings you actually see all depend on ambient temp of course. The colder it is outside, the closer you would be to 20 psi. This would be if you run the defroster on a 40 degree day for example. If you are running the AC on a 95 degree day, you'll probably be closer to 30 psi on the low side.

There are some temp to pressure charts available if you search. As you can see, you are already as low as 25psi in some of these scenarios. If you don't plan on doing much driving and using the AC in cooler weather (for defrost) then 28 psi might not be an issue.