Your requirement is step down conversion (20VDC to 8VDC)
t on/t off = duty is the duty cycle ( Duty = 50% or more generally )
fmin is the minimum frequency
Vf is the diode forward voltage drop (.5V)
CT is oscillator frequency setting capacitor you can vary the PWM frequency by varying CT value
V ripple is the ripple voltage present in the output, you can have decided the ripple by varying Co(output capacitor)
L min is the minimum inductance required
Hope it will help you
Read the data sheet carefully everything is given