Pressure is dependent on ambient temperature. The re-charge can should have a temp/pressure chart to go by, but 35-40 low side (as stated) is good rule of thumb. I would also recommend using a thermometer on the upper air vent which will also help to know when to stop adding R134. Open front doors and use recirc so that engine heat won't give false ambient temp. Fan speed should be about half speed. But be patient and fill slowly a bit at a time and let the system settle before determining if you need more refirgerant.

92 Montero LS 3.0L V6 Auto, Stock, Original owner, 185,800K miles