Take the tensile strength times the cross section area for the maximum load (?). I recall stress being force per unit area. So stress times the area should give the force.
For example, the tap drill for a 3/8" x 16 is 5/16" or a diameter of 0.3125". Pi x D / 4 is the area or 0.0767 sq. inches. For a 100,000 psi material that'd give a load of 7670 lbs. I'm assuming no stress riser from the thread (like a sharp notch). A fine pitch 3/8" x 24 bolt has a larger cross section so it would break at a higher load.
As I recall from my mechanics class, tensile strength is the top of the curve for stress vs. strain. The material has an elastic region where it deflects proportionally to the load. Then it starts to permanently deform (plastic region). When the deformation (strain) is a 0.2% of the original length, that load (stress or force/unit area) is the yield strength.
The curve knees over at that point and reaches a peak (tensile strength). At which point it starts moving downhill until it ultimately fractures. Google stress vs. strain diagram for more info (probably already more than you want to know . . .).
Or ignore most of what I typed and go for the grade 8 bolts recommended above!
Bruce