647,000 gal in 30 days is approximately a continuous 15 gallon per minute flow rate. Flow rates involve more variables than just pressure ... so there is no direct conversion between the two. Also, pressure is not a constant, it varies depending on water demand from neighbors, pipe friction, etc.
My advice to you is to get a 5 gallon plastic bucket and time how long it takes to fill the bucket at your maximum flow possible. If it takes about 20 seconds to fill it, then the utility company is correct. If it takes more time, then your flow rate is obviously less than that.
say your it takes "t" seconds to fill the 5 gallon bucket; your flow rate can be calculated as follows ...
so, divide 300 by the number of seconds it takes to fill the bucket to get your flow rate in gallons per minute.
for example, lets say it takes 35 seconds to fill the 5 gallon bucket ...
to convert gallons per minute to gallons per 30 day period, multiply the gal/min by 43200 (the number of minutes in a 30 day period).
so, if it is say, 8.6 gal/min , that rate of flow would yield 8.6 times 43200 = 371,520 gals
hope this has been some help ... you also might look up a company that deals in irrigation and speak with someone more knowledgeable in this subject.