Generating PWM from rectified sine wave using 555
The highest voltage into R16 will be at the peak = (20V - 0.6V) = 19.4V

The lowest threshold will be zero. Unfortunately the threshold will be zero the entire time that the sine is &lt; 0.6V.  So there will be a portion of the wave that the PWM output is off, or undefined. To solve that, you might want to add some additional current into R17 (an additional pullup resistor).

But solving with what we have, we want Vthreshmax such that pulse length is ~120us. (80% duty cycle)

Vctl = Vcc * (1- exp(-t/RC) ) 

Vcc = 15V, t = 120us, R = 10kohm, C = 10nF, 

Result: required Vctl = 10.5V

R17/(R16+R17) = 10.5/19.4 = 0.54

R17 = 0.54 R16 + 0.54 R17

0.46 R17 = 0.54 R16

R16 = 0.46/0.54 R17 = 0.85 R17

So, if R17 = 100k, R16 = 85k

Please check the math :-)