The spectrum of the light has a lot to do with your measured "speed," since graded paper is UV/blue sensitive only, and multigrade also has a bit of green sensitivity, but is also contrast-sensitive to the color of light.
I've also found that for paper being used as an in-camera negative media, you want to develop to completion in order to get adequate highlight detail without a mottled and streaked appearance. This differs from negative film which, within limits, can be given various development times in order to affect the resulting contrast.
For control of contrast in high-key sunlight I've taken to using grade 2 paper rather than multigrade, since it's much less contrast sensitive to the color of light. I also apply a preflash exposure to the paper, to increase shadow detail and therefore further tame the otherwise excessive contrast.
I've found it also wise to apply consistent developer dilution and temperature when developing paper as an in-camera negative; this helps to reduce process variability.
~Joe
Here's a recent example of Arista's grade 2 paper, post-flashed just prior to development (works about the same as preflashing), shot in an 8x10 pinhole camera at the City of Rocks state park in southwestern New Mexico. I rate this paper at an exposure index of 12, and develop it in Ilford Universal paper developer mixed 1+15 at 68f. I metered the brightest part of the boulder and placed it at +1 stop; exposure time was around 35 seconds, IIRC.
