What are the "magic numbers" on a Monoprice Select Mini? What are the "magic numbers" people refer to regarding print resolution on the Monoprice Select Mini?
The "magic numbers" are optimal values that work particularly well for the layer height.  Michael O'Brien derived these numbers by reverse engineering the mechanics of the Z-axis stepper motor.

Using these values as your layer height will generally improve your print quality over using round layer heights such as 0.15, 0.2, or 0.25 by eliminating quantization errors.

To see an example of this, print a copy of 3DBenchy at 0.15 and 0.175.  On the 0.15, you will see some wavy patterns on the curved bow portion compared to the 0.175.  This is the result of inexact rounding.

Layer Height (mm)
0.04375 (results may vary)*
0.0875
0.13125
0.175
0.21875
0.2625
0.30625