There's a fantastic set of two textbooks covering bolted joint design by John H. Bickford. One for gasketed assemblies and another for non-gasketed assemblies,

There's a fantastic set of two textbooks covering bolted joint design by John H. Bickford. One for gasketed assemblies and another for non-gasketed assemblies, if you want to really _really_ dive deep into it. At the end of the day you get the final clamping force by multiplying a bunch of modification factors together that have been empirically measured and the bolting procedure has been verified. When you step outside any of the parameters you've verified in the past, you have to substantiate the math with experimental proof. The size of the fastener, fastener style (e.g., stud vs. bolt), fastener length, thread pitch, torquing method, presence of a washer, presence and type of lubricant, fastener material, base part material, and bolt arrangement ALL play into the final bolt assembly procedure. A lot of these are well-known and published. Nobody is out there on assemblies like this just slinging unsubstantiated numbers around, at least if they're doing their due diligence.