The truth of the matter is that BOTH high and low has to be looked at. The pressures have to be corrected for air temperature.
For example, let's say that the high side reads 281 PSI. If we look at the table, that gives a state change temperature of 155 degrees F.
Here where the outside temperature gets important. For this example, let's assume it's a very mild 50 degrees outside. This give a delta T of 105 degrees. This is way too HIGH and almost certaintly indicates a problem.
Now let's say its 100 degrees outside. Add say 10-15 degrees for engine heat and now the Delta-T is 40-45 degrees. This is a more reasonable Delta-T.
The high side pressures have to be monitored to protect against overcharging. However, the "normal" pressures are going to depend upon what the outside temperature and air flow is.
That is why it is vital to clean the coils and make sure that all fans are working correctly. Reduced air flow will skew the pressure readings!
If you absolutely can not monitor high side pressures, then monitor the low side but do not let the low side get above say 45 PSI. Add Freon until cold air starts coming out. If you get to 45 PSI and no cold air is coming out, best to stop and find out why. It's just not wise to go higher unless the pressures are crossed checked against temperature or you understand WHY the pressures are elevated.
Disclaimer. Working on AC systems can be dangerous. Serious personal injury can result from improper operation.
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.