Car A/C pressure fluctuates when refilling?

There should be a sticker with the max fill for the a/c system. I wouldn't recommend trying to recharge if it is giving you trouble the pressure on the high side can explode with deadly force. If having to refill then has a leak the system doesn't use up the freon. If the leak is great enough your compressor will not stay on give you and accurate pressure reading. If a newer car shouldn't take much more than 16oz to keep the clutch engaged.