That is the correct formula for converting _spherical_ candlepower to lumen.
The spherical candle power ( or mean spherical candlepower) is a measure of the total light output of a source. It is the candela measurement (candlepower) averaged over all directions (the sphere). 1 MSCP is the light output of a source with an intensity of 1 candela over the entire sphere.
The lumen is a measure of the total light output of a source. It can be directly converted to MSCP. 1 lumen is the light output of a source with an intensity of 1 candela over a solid angle of 1 steradian. A steradian is a measure of solid angle (think cone), and is exactly 1/(4*PI) of a sphere.
Note that the above formula will explicitly _not_ work for converting _intensity_ to light output. It doesn't convert _candlepower_ to lumen, but instead converts _average candlepower_ to lumen. A spotlight with a very narrow, focused beam might have very high _candlepower_ in the center of the beam, but low _average candlepower_, and thus low lumen output.
-Jon
