Sagging or gapped top layers usually trace back to one setting
A top surface that shows small gaps, a slightly rippled texture, or visible infill pattern ghosting through is one of the more common and most fixable quality complaints in FDM. The top layers need to fully bridge over the infill pattern below, and a wider unsupported span simply needs more material closing over it before the surface reads as solid and flat. This tool starts from your actual span and cosmetic priority rather than a flat default top-layer count.
How the plan works
A recommended top layer count scales with span: a narrow span needs relatively few layers to close cleanly, a wide one needs more. A cosmetically important top surface adds one more layer of margin on top of that baseline, since a slightly-too-thin top surface is more forgivable functionally than visually. Your current setting is compared directly against this recommendation.
Worked example
A 20 mm span, cosmetic top surface, currently 4 top layers: the tool recommends about 6 layers for this span and cosmetic priority, flags that 4 is likely insufficient, and offers increasing infill density under the span as an alternative if adding two more top layers is not desirable.
A common mistake
A frequent error is adding top layers to fix a sagging surface without checking whether the infill underneath is actually the real limiting factor. If infill density is very low, even a generous top layer count has little to bond to and can still sag; check infill density alongside top layer count rather than treating them as independent fixes.
Limitations
This estimates a reasonable top-layer starting point from span and appearance priority; it does not know your specific infill density, pattern, or cooling setup, all of which affect the real outcome. Check the result in your slicer's layer preview and adjust from there.