You can't accurately charge a system by pressure alone. To wing it with an expansion valve system you charge by subcooling. Get the chart listed above and learn to do subcool and superheat calculations. You have to know where your refrigerant is and what it is doing. If you do not have liquid where you need liquid and vapor where you need vapor your system will not work well and you may damage it. The correct way to charge these systems is by weight but you can wing it if you do your homework.

Hello from the Mill Mountain Coffee and Tea shop in beautiful downtown Roanoke, VA. Will shortly be on the trail and out of contact.

Run some searches here for more comprehensive information on these systems.