That's only useful if you're building your own DPSS laser (or modifying an existing). I wouldn't want to use it with a direct injection laser.
If you do, you must consider that the wavelength changes with temperature. Also that longer wavelengths have a higher photon flux at a given radiant power, but at the same time Rayleigh scattering will be more efficient at shorter wavelengths. Looking at the relevant equations I get that the wavelength dependence of the photodiode current is (λ/λ)^3.
That is of course assuming there are no particles in the air. If there is, Mie scattering will dominate and the equation becomes nearly impossible to solve.
For a DPSS laser it is of course completely different, but what you are talking about here is close to building a whole driver circuit. Even so, I don't think you should use 'spill' light; the error will simply be too big unless that photodiode is inside of the laser module, which must be hermetically sealed and dust free. I am of the opinion that the sensor must be in the beam.