Calculating Extents in the Field Calculator

Here's a little shortcut.  You can calculate extents in the Field Calculator dialog using:

dim Output as double
dim pGeom as IGeometry

set pGeom = [shape]
Output = pGeom.Envelope.XMin

Choose advanced, paste what is above and set your field equal to Output. 

NOTE: If you want to add the values in Decimal Degrees or Web Mercator there's an app for that.  Ask Tim Williams how to install his Calculate Extent button.