I made some rough measurements, using the setup I had used for CCD calibration, to get an idea of how good of a Lambertian scatterer the white paper is. Following are the values I got:
Note: All the measurements are just rough ones and are prone to larger errors than estimated.
I also measured the transmittance of the white paper sample being used (it consists of 2 white papers wrapped together). It was around 0.002