Potential rainwater harvesting calculator, the numbers don't seem right, what am I doing wrong?

The rainwater harvesting calculator at http://pccd.org/rainwc.htm, at least from my understanding states that for every 1000 sq ft of roof you can expect to collect 550 gallons of water for every inch of rain that falls.
To convert that into a formula I can better understand and use I went through the following steps.
(Roof area / 1000) x 550 x (Annual rainfall in mm / 25.4)
Our roof area is 1400 sq ft and where we live the annual rainfall is listed as 1275mm. This makes the formula;
(1400/1000)*550*(1275/25.4)
1.4 x 550 x 50.2
The final answer is 38,654 gallons/year

That seems like a an awful lot of rainwater we could potentially collect, like way too much. 743 gallons a week on average.

Is my thinking and therefore my calculation correct or have I messed up on the math somewhere?

I think your math is right.

I did the numbers and came up with the same figure of 38,654 gallons annually. I think it's truly amazing how much water can come off a roof with good rainfall, even a small roof. It's a huge amount.

simple calculation of 1400 square feet x (1275 mm / 25.4 / 12) = 5,856 cubic feet of water, or 43,800 gallons per year.  Their slightly lower number probably accounts for minimum wetting rainfall, first flush losses, etc.  

But you're right - it's a staggering amount of water.  I had an 1,800 gallon rainwater tank when I lived in Arizona - not exactly a rainforest environment.  I could water my trees as much as I wanted, and that tank was always full by the next rainstorm.

I get a chuckle when I see people connecting a puny 50-gallon barrel to their downspout.  They're clearly have not done the math!

Thanks folks!!! I thought the math was right but the numbers just seemed too high.