In response to a question in the Schedule PSD thread in the AutoCAD Architecture 2008 Discussion Group, I posted a file done in ADT 2007 that demonstrates that an Anchor property can be used in a Property Set Definition attached to to a Window object to read in property data from the Wall object and use that to calculate the necessary window opening.
Two different options are given, one that assumes a style-naming convention that has the name of all stud wall styles starting with the letters "stud", and another that assumes that a manual property attached to the wall style contains the string "STUD" for all stud walls. The formulas associated with each method pass through the value of the automatic WidthUnformatted property for stud walls and add 3" to the width for all other wall types.
Use of a Anchor property, introduced in the 2007 release, makes this task much easier. Previous releases would have required a formula property to dig through the drawing database, to find the associated value.