Layer heights that divide evenly into the Z axis full-step distance are called magic numbers. They matter because a stepper holds a full-step position far more firmly than a microstep position, and a layer height that lands between full steps makes every layer slightly different.
Where the number comes from
Divide the screw lead by the motor's full steps per revolution. A 1.8 degree motor is 200 steps; on an 8 mm lead screw that gives 40 microns per full step.
So on that very common setup, magic numbers are multiples of 0.04 — 0.08, 0.12, 0.16, 0.20, 0.24, 0.28. The popular 0.15 is not one of them; 0.16 is.
On a 2 mm lead screw the full step is 10 microns, and almost every sensible layer height divides into it. Machines with 2 mm lead Z screws essentially do not have this problem.
Why microsteps do not save you
Microstepping positions the rotor between full steps by balancing current between coils. The holding torque at those intermediate positions is a small fraction of full-step torque — at 16× it is a few percent at the first microstep.
The Z axis carries the weight of the gantry or the bed, and that load pulls the rotor toward the nearest full step. So a commanded position between full steps is not reliably held.
The result is not a dramatic failure. It is a fraction of a layer of inconsistency, repeated up the whole part, which the eye reads as banding under raking light.
How much this actually matters
Honestly, less than the internet suggests. On a well-built machine with a 2 mm lead screw, magic numbers make no visible difference. On an 8 mm lead screw with a heavy bed, they do.
The cost of using them is nothing — you pick 0.16 instead of 0.15 — so there is no reason not to. It is the cheapest print quality improvement available.
If you see banding at a regular interval that does not correspond to anything mechanical, this is one of the first things to rule out.
Layer height against nozzle diameter
The other constraint has nothing to do with steps. A layer thicker than about 80 percent of the nozzle diameter does not get squashed enough against the layer below, and the bond between layers weakens.
Below about 25 percent, the extruder is metering a very thin ribbon and print time climbs steeply for little gain. Most work sits between those.
For a 0.4 mm nozzle that is roughly 0.1 to 0.32 mm. The calculator reports where your choice sits in that window.
First layer is a different question
The first layer is usually thicker than the rest, often 0.2 or 0.3 regardless of the layer height above it, because a thicker first layer is more forgiving of a bed that is not perfectly flat.
It does not need to be a magic number — it is one layer, and any inconsistency in it is absorbed by squash against the bed.
What matters for the first layer is squish, set by the Z offset, and that is found by printing a single-layer square and adjusting live rather than by arithmetic.