Core idea
Cutting speed depends on work material, tool material, coating, rigidity, coolant and desired tool life. Start from reputable tooling data, then tune conservatively.
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.