Layer height is a dial, not a single correct setting
Every layer-height choice is a tradeoff between visible surface quality and print time, and the right answer changes with the part. A display piece with curved surfaces shows every layer line at a tall layer height; a bracket that lives inside an enclosure does not care. Meanwhile, layer count, and therefore most of the added print time, scales roughly inversely with layer height: halving the layer height roughly doubles the layer count for the same part. This tool compares three candidate heights relative to your nozzle rather than suggesting one universal number.
How the candidates are chosen
Fine, standard, and draft candidates are computed as fractions of your entered nozzle diameter (roughly 25%, 40%, and 70%), which mirrors how most slicers scale their layer-height presets to nozzle size rather than using a fixed value regardless of nozzle. The tool then picks a best-fit candidate from your stated detail target and time priority, and shows the other as a genuine alternative with an explicit reason you might still choose it.
Worked example
A 0.4 mm nozzle, visible-surface target, medium time priority, 50 mm tall part: the tool recommends the Fine height (about 0.10 mm, roughly 500 layers) as the best fit for visible quality, with Standard (about 0.16 mm, roughly 313 layers, about 1.6x faster) shown as the alternative if the extra print time is not worth the finish improvement for this particular part.
Limitations
The layer-count and relative-time figures here come from geometry and layer height alone; they ignore print speed, acceleration, travel moves, and your specific slicer's own time estimator, so treat them as a comparison between options rather than a quoted print time. Cooling performance and material also affect how well a given layer height actually bridges and cools, independent of the height itself.