Hi,

In the circuit attached, the author says: "By varying the pin 6 voltage, the on/off points are moved up and down the triangle wave, producing a variable pulse width. Resistors R6 and R7 are used to set the end points of the VR1 control, the values shown allow the control to have a full on and a full off setting within the travel of the potentiometer. These part values may be varied to change the behavior of the potentiometer."
Reference: http://www.solorb.com/elect/solarcirc/pwm1/

However, I do not want full on. I want the max duty cycle to be 50%. How can I set those resistor to achieve that?

 

I want the max duty cycle to be 50%. How can I set those resistor to achieve that?

Increase the value of R7 so that the maximum duty cycle is 50% when the pot is at the bottom of its rotation.
You can determine that value experimentally.

Alternately you can replace R7 with another pot connected as a rheostat to adjust for the maximum.