How do I find voltage value at the a node point in the schematic?
The idea here is that the chip gets (limited) power trough Rstartup initially, but when it starts working, the separate winding of the transformer (Vaux) provides (more) power for the chip. In any case, the zener diode is there to limit the voltage to 18V.

EDIT in response to the comment:

If Vx becomes higher than 18V above ground, the zener diode starts to conduct and the current trough Rstartup would cause a voltage drop on it and would cause Vx to go back down to 18V.