Hold the phone in the intended orientation for the stand (usually portrait format) measure its dimensions (mm) and put them into the appropriate fields in the Customizer window.
For most people that is enough customization.
The other values affect the shoulder widths between the phone and the base edge, rounding off of the sharp edges and the aesthetic of the ratio between phone height and base height. Feel free to play with them but they can be safely ignored.
P.S. Don't forget to measure with the silcon cover in place (if any).
P.P.S. It is also a good idea to inflate the measured values by a couple of millmetres to get an easy fit.

Creality

Ender 5

No

No

10%

The design of this Thing is basically two intersecting cubes.
The first cube is the phone itself,
The second cube is the base.
The width of the base is the width of the phone plus the (parametric) width of Left and Right shoulders.
The height of the base is derived from the (parametric) ratio of the base height to phone height.
The depth of the base is calculated using high school geometry from the height of the phone and the (parametric) leaning angle.
The location of the phone relative to the base is (parametric) height of the bottom of the phone from the floor of the base and the (parametric) distance between the front of the phone to the front of the base.
By slanting the phone, locating it in the base then removing it we get the basic product.
Next we cut away excess material and save printing time by defining a third cube and subtracting that from the base.
Finally we round off the sharp edges using a (parametric) fillet radius.