1) Draw a 
horizontal line at bottom of the arch (the black dotted line) 
 
  2) Measure 
the bottom Horizontal (B measurement) to the ends 
of
      
where the casing will begin and end
 
  3) Measure 
the Mid Height 
      
The rise from the horizontal black dotted line 
      
to the top of the arch (the A measurement)
      
Measure the inside of the arch.  
  
  4) Measure 
the Incremental Heights
      
The various rises from the horizontal black dotted line 
      
at various horizontal increments say at 12" apart 
to the top of the arch. 
      
Measure the inside of the arch   
 
  5) Are you 
doing one side of the window or doorway or both ?  
 
  With these 
measurements we will calculate how many feet you will need. 
 
  The minimum radius 
is approximately 3 times the width of the moulding, however
  this varies with 
the thickness of the material (ie depth).   
  Check with us for 
the minimum radius for your application.