Hi Mike,
indeed, the FSX/P3D world is divided into a regular, nested grid; see http://www.prepar3d.com/SDKv4/sdk/world/terrain/terrain_overview.html#Base File Information
Using that map it's relatively easy to find the name of the numbered default folder for your area of interest. However, those folders are based on the QMID 4 level (the red lines in the map) while both the default and Vector's file sets are based on the smaller QMID 7 level (the black lines), which is the column/row number you need to determine.
If you have ADE (Airport Design Editor) installed you can locate a nearby stock airport and the selection screen will also list the default file it's in, which contains the local grid cell number in its name. For example, start ADE, search for city Detroit, then select KDTW, and its file name gets listed as "APX25160.bgl" meaning you're looking for QMID 7 grid cell "2516".
Alternatively, if you have the P3D/FSX SDK installed, you can open its TMFViewer.exe tool, activate the QMID7 grid display, and then use Jump > Lat/Long to center the display on the local grid cell and to read its number (File#) from the bottom of the screen. The local coordinates can be copied from Google or Bing Maps or the sim's map or coordinate displays.
Cheers, Holger