1) Draw a horizontal
line at bottom of the arch
(the black dotted line)
the bottom Horizontal
(B measurement) to the ends
where the casing will begin and end.
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.
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 ?
calculate how many feet you will need.