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Seated Beam Connections

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I would like some asistance to resolve a design question.  I am designing an angle used as a beam seat.  I checked my college steel book and Gaylord was using Fsubb = 0.66Fsuby. I checked some more sources and Salmon and Johnson was using 0.75Fsuby, Fisher's Vulcraft book was using 0.75Fsuby.  Which one is right???
With a little more looking I found in the ASD Specs F2.1 (Compact) Fsubb = 0.75Fsuby or F2.2 (Noncompact) Fsubb = 0.60Fsuby.  These sections refer to Section B5.  Is the appropriate test b/t < 76/Sqrt(Fsuby)???
Finally, My real question, this angle is not a single beam seat but a ledger angle supporting multiple purlins or bar joists.  What is the effective length of support to subsitute into fsubb = (6*M)/(lsube*t^2)???  Surely it is not the 5' spacing of purlins, the 2' spacing of bar joists, or the spacing of bolts or welds which attach the ledger to the wall.  How does one determine the effective length of support on a continous ledger angle???
Thanks in Advance
Jim Hannah, EIT
Butler Manufacturing Company