Ok, Siffy. Being a student, here is the original source of information I used and continue to use when looking at lumber in a stand setting.
Full Document portal:
http://www.fpl.fs.fed.us/documnts/fp.../fplgtr113.htm
Specifically, Chapter 4: Mechanical Properties of Wood
http://www.fpl.fs.fed.us/documnts/fp...tr113/ch04.pdf
For those with the background, the number I typically employ within my beam bending calculator is 4000 psi. A piece of Douglas Fir has a rupture strenght of ~7000 psi when GREEN! This "Strength of Rupture" is determined experimentally and considered the working value for lumber under bending loads.
As to what deflection is acceptable, personally I work for 1/8th inch and this is typically more restrictive then the strength of the beam.
Should anyone have any questions, let me know. I will be happy to post the equations I use for those interested.