Core idea
Bolt preload is the tension created when a fastener is tightened. The joint is held by clamp force, not simply by the bolt resisting sideways movement. Friction has a major effect on torque-to-preload accuracy.
Practical approach
Confirm units, identify the governing standard or manufacturer data, record assumptions, calculate an initial value, and check whether the result is sensitive to friction, temperature, efficiency, tolerances or material variation.
Use calculators with judgement
Online tools are useful for comparison and first-pass design. Safety-critical work requires suitable factors of safety, verified inputs and review by a competent person.