Are you asking how to:

• "measure" torque?
• calculate torque?
• apply the proper torque?

First of all, you're jumping ahead of yourself. To even begin considering how much torque to apply, you have to know how tight the fastener must be. Once you've determined the bolt stress requirement, you can begin to guess (all torque calculations are based on a "guess") how much torque might be required to develop the necessary bolt load.

Hi,

To measure the torque we can use the torque wrench. I'm asking how to calculate and apply the proper screw torque that should be applied.

With all due respect, your approach is the one taken by most people. Unfortunately, it's misguided and wrong

You can't even think about trying to determine a target torque before you know how tight the bolt must be! Once you have that figure, multiply it by the fastener's nominal diameter, take a WAG* about what the K factor might be, multiply the latter by the results of the former and. voila: Torque (if your units are cleaned-up!)

* Unfortunately, unless you can see into the future, it will be impossible for you to know what the actual K factor will be during assembly and all future re-assemblies. This means that the torque value that you've calculated likely won't result in the bolt load that is required.

Now, if you're asking how to calculate bolt load, that's an entirely different matter: You need to do a complete analysis of your joint. Things to consider are internal loads, external loads, pressure, temperature, environment, joint materials, bolt materials, nut materials, gasket characteristics, joint stiffness, bolt stiffness, joint geometry and a host of others.