This takes moments about the lower right hand edge of the wall, labelled A in the diagram below. If the 'restoring' counter clockwise moment, due to the weight of the wall, exceeds the 'overturning' clockwise moment caused by the wind load, then the wall is stable.
The factor of safety is the restoring moment/overturning moment.
The restoring and overturning moments are per metre of wall length.
David Grieve, 20th May 2005.