I believe you are correct. You have to wait 30 days until you can file an inquiry. TSC then has 30 days to respond.
Most people would assume that if your ND is 5/2/02, then you would have your GC by today. This obviously isn't the case.
Assuming your ND is 5/02/02, you would have to wait until 11/6/04 to file the inquiry. TSC has until 12/6/04 to respond. Assuming they give you your FP notice on 12/6/04 it seems that most appointments are 30 days from the date the notice is issued which brings us to 1/6/05. It will likely take until 2/6/05 for the fingerprints to make it from the local office to the FBI and then to the TSC. Let's just say they approve it at the end of this month. Therefore if your ND is 5/2/02, you will most likely have to wait until 2/6/05 before you are approved. We all know this isn't the case because approvals seem to be entirely random.
Frustration