calculation for tightening torque

You'll be limited by the pull out strength of the aluminum.

See my post #32 in this thread for his to calculate it.

Why reduce it?

With BJA you're primarily juggling the 1. number of fasteners, 2. diameter of fasteners, and 3. diameter of washers to ensure you're A. stretching fasteners to 75-90% for durability, B. without yielding material underneath your bolt head, and C. providing the necessary clamp load.

If you're stripping threads then your fasteners are too large. If you're yielding the material underneath the bolt head then you need a larger bearing area (washer). If you need more clamp load then you need more and/or larger fasteners.

If your shop has a standard set of torque values that they use when prints dont callout specific torques, you should be using that to communize print callouts AFTER running a BJA, but otherwise engineers should never need a torque chart. Usually that indicates somebody's guessing rather than engineering, which inevitably causes failures and hopefully terminations for cause.

The first question I would address is whether the loading is dynamic or static, and how many stress cycles. The second is what safety factor you need for your application. A third is what kind of statistical variation in strength the bolt exhibits (this may be built into proof load). The bolt supplier can be a good resource for your question.

In addition to the considerations Doug Hunter states, it would also be worth considering how often this joint will be disassembled and reassembled. If that will be frequent, then you should consider adding a threaded insert to the tapped hole (to reduce the risk of galling and other issues with tapped holes in aluminum). If a threaded insert is used, that will change your joint capacity calculations, so best to make that decision early on and avoid redoing any calculations.

If you are providing more than two diameters of thread engagement the aluminum thread "strength" may not be a problem.

This is related to post 3 CWG1's second paragraph.