How does it choose a suitable led power?

For the “less than” 40V LED input voltage, if output voltage is less than input voltage, please choose that topology construction of reduction voltage.

and it will make that work easier. The following we will choose the Zener diode. In other words,the Zener diode match your circuit base on input current. If input current is big than 1.2A. it may be use the Zener diode of high bright LED to supply input current. The zener diode have minimal fed back voltage.it seems perfect for steaby current work that LED average current.