How to calculate EXTENT

[Christian]

Hello,
I am rather new to mapserv and have even not much knowlege about GIS. For 
my first experiments, I got a shapefile from a colleague. It took me some 
hours of trying around and to find the right numbers for the EXTENT line in 
the mapfile.
Accidentially, I found some numbers that gave me at least a viewable map. 
But compared with those numbers in the demo-files, they are really strange:
	EXTENT 22 45.0 -2 57
As said, I found this just by typing more or less random numbers and then 
using shp2img to get a gif-picture. How can I calculate the EXTENT entries 
for my maps that they are visible?

Many thanks!
Christian
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christian