How to calculate output voltage of a non polarized capacitor?
The capacitor and resistor (load) form a voltage divider. The voltage "out" of the capacitor depends on the value of the capacitor and the value of the load (resistor). It is impossible to calculate the voltage after the capacitor without knowing the load resistance.

The "tutorial" you cited is rather a poor example. A capacitor is NOT "like a transformer". It is much more like a resistor in this application. Furthermore, none of the examples or calculations account for the load resistance (R). Although in the table you asked about, they DO state the CURRENT that they are drawing.

If you want to make a direct-mains powered device like this, you need to construct the "load" part (the LED or whatever) FIRST and measure the voltage and current required to operate it.  THEN you can determine what size dropping capacitor will be suitable to power your circuit.

Note that working on direct mains-powered circuits is DANGEROUS and proper precautions should be taken. If you don't know what "proper precautions" are, then you probably should not be attempting such dangerous activity. At least without some more experience behind you.

Recommend a YouTube guy named "BigClive" who has dozens of videos "tearing down" mains-powered LED lights, etc.  He explains how they are made, traces out the circuit, and points out the dangerous bits.  At least watch a dozen or two of his videos before tackling anything direct mains powered.

Ref: https://www.youtube.com/channel/UCtM5z2gkrGRuWd0JQMx76qA