Hi Jereme. I just happened to be torquing a screw on a receiver rail last week of this dimension, so I had it fresh off the top of my head. Keep in mind that this was a lubricated thread, so a dry thread should be about 30% extra torque.
I too have really struggled to find legitimate sources for torque specs. When I can't find something applicable in aircraft specs I will fall back on SAE values, however even these don't seem to be published down to the values we use. Thus what I have been doing it using torque values provided by screw manufacturers and suppliers, then comparing material types and tensile strengths against what are published for AN507/AN509 screws. Below is the best chart I have found to date. Note the dry thread/wet thread difference that you need to apply, however in most situations for us we are applying dry threads anyway. As an additional note, I'm a huge fan of that torque screwdriver I have. It is often more convenient to use than the torque wrench on AN3 bolts due to the lack of a handle at right angles. Definitely one of my most used tools.
Good luck.
Tom.
http://www.federalscrewproducts.com/torque-chart.htm