Pat,
Here's a link to the US Coast Guard's Simplified Measurement Guide I mentioned.USCG Simplified Tonnage Guide
You can use this for vessels under 79 feet. As Olivier points out different countries have different methods.

You should work it out yourself, but, for your SM it would be ABOUT the following (I had to guess at a couple of numbers but you can fine tune it.
GRT = (Hull volume + Deckhouse Volume) / 100
Where Hull Volume = S x K x L x B x D and Deck Structure Volume = Ls x Ds x Bs 
L, B and D are overall length, breadth and depth, but depth is measured from the deck edge to the bottom of the keel.
You consider the Deck Volume to be 0 if it is less than the hull volume, which it is.
S is a Shape factor which is 0.5 for sailboats. K is a Keel factor which is 0.75 for our shape of sailboat.

So, let's use Richard Colebatch's posting for your SM dimensions. That is 52.49' in Length and  15.09' in Breath.
Richard shows a Water Draft of 6.73' from the bottom of the keel to the water line and let me guess another 6' of freeboard to the edge of the deck.
That would be a Depth of 11.73' (which seems about right as my Santorin's Depth is 10.5')

So the SM Gross Register Tons (GRT) would be (they say to round feet to the nearest tenth):
  S  x  K    x  L     x   B    x  D   
0.5 x 0.75 x 52.5 x 15.1 x 11.8  = 3507  / 100  = 35 GRT
Then, NRT = M x GRT, where M is a Machinery factor which is .9 for sailboats, so
NRT = .9 x 35 = 31  (you are instructed to truncate any fraction)

So, it does seem your numbers are, indeed, way high and should be more like a GRT of 35 and NRT of 31.

Bill Rouse's GRT 27, NRT 24 comes from his Documentation Certificate, I'm sure, but seems low as my Santorin is 31 and 28 and the USCG calculation above is pretty straightforward. Anyway, there are lot of other numbers in this thread that may have their origin in countries other than the US and/or may be weight based. Numbers that have fractions are definitely not US GRT/NRT, which are always whole numbers. Also, in the US the documentation is pretty much self reported with, seemingly, very little checking by the NVDOC. Whatever, your's seems high.

The US Coast Guard's National Vessel Documentation center has a super new website that it very easy to navigate and you may be able to change your numbers on-line. Check it out.

Cheers, Craig Briggs, SN68 with  GRT 31 NRT 28