I'm not a fan of the DIY systems. Refrigerant is usually low because of a leak, so not only will the new refrigerant probably leak out, too, but air and moisture probably got in, so the system really needs evacuated at least, and the drier probably needs replaced. Another issue is now much to add. You don't know how much is in there, so adding more is a total shot in the dark, too much and the system won't work right, either. A single gauge on the low pressure side doesn't even come close to giving you the whole story.
