There is no one answer to your question, for example:

I have a slope in the middle of a farm field, it moves 6-inches per year, I don't have a problem.

OR

I have a slope with a 3 story parking garage on spread footings at the crest.  It moves 1/4 inch per day for a week, I have a very big problem.  

All depends on what the slope is doing and what is in the area.  By the way the parking garage example is real, just not my garage, :).

What you are trying to do is not determine a "factor of safety" which by definition is based on shear (strength) parameters, but determine a servicability limit.  As GeoPaveTraffic says, what level of movement you can permit will be based on the permissible movement limits of your slope with respect to itself, or to buildings on top, or pipelines within, etc. What you would do it show plots of movements (at selected) locations given the site geometry, soil stratigraphy/parameters, groundwater level, etc. - like a senstivity analysis.  Then you can select the geometry (i.e., slope angle, need for berms, etc.) based on your project needs.

I guess I'd just add, what is the nature of the movement?  If you have a temporal condition that brings about a few inches of movement, that could be a sign that the safety fractor trended toward unity.  Next time if that temporal condition develops you may not be so lucky (i.e., catastrophic failure).  Then again, newly-constructed slopes can also realize movement owing to compression or consolidation related to vertical stress (v. shear stress).

Just something else to ponder. . . .

f-d

¡papá gordo ain’t no madre flaca!