Core idea
Metric threads are described by nominal diameter and pitch. Unified threads are usually described by diameter and threads per inch. Similar-looking fasteners may not be compatible.
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.