How to set the boost output voltage in this IC?
You don't set the voltage.
It is an LED driver.  You set the current, and the voltage takes care of itself.

Set the current using \$R_{VFB}\$ as described in equation 5.  For 300mA, it looks like you'll need a one ohm resistor.
As long as the sum of the forward voltage for all of your LEDs in series is less than 60V, it'll work.