>>I don't understand you. If that condition was in the original SQL Statement, do we want to preserve it or not? I assume that this condition does a rough filtering and then distance filtering is more precise. If this assumption is correct, then we may want to use this 'rough' filtering first to get less records and then apply more precise filtering based on the computed column.
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>Please, note also that those SQL are to support SQL Server 2005. In SQL Server 2008, with those spatial data types, the SQL for most of those operations get much simpler. So, as I need to support SQL Server 2005, I am using those ones for now. But, once they will move to SQL Server 2008, they will be able to use the other one.

Right, I understand. Basically, I always referred to this blog post
http://blogs.lessthandot.com/index.php/DataMgmt/DataDesign/sql-server-zipcode-latitude-longitude-pr and the other one by Denis Gobo.

I re-read that blog again today, but I don't see actual good explanations for the constants used in that article. I saw a request for explanation in the comments, but that was it.

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The calculation in that article is in miles. What exactly has to be adjusted for km is not 100% clear.