Pressures should be pretty close to the same on any system, as pressure is directly related to temp. A wise old dealer mechanic told me that the only way to get a proper charge in any automotive system is to fill it from a full vacuum by weight. Unlike a big commercial or even home HVAC, an automobile AC has a very small quantity of refrigerant. Adjusting by pressure is not nearly accurate enough to avoid under or overcharging by a few ounces, which is a pretty significant amount in such a small system. Unless you are seeing a real difference in pressures (blockage in the expansion valve or such), best to not adjust unless it is bad low or high, overall. If you have to evacuate, fill to specified quantity by weight and go.