I need to vary the duty cycle between 50% and 70% of a PWM signal
Work out what your sawtooth minimum voltage is. Let's call that Vmin.
Work out what your sawtooth maximum voltage is. Let's call that Vmax.
Now work out the voltage for 50% (half-way) between Vmin and Vmax. Let's call that V50.
Then work out the voltage for 70% between Vmin and Vmax. Let's call that V70. 


You've chosen a 1 k&Omega; pot so now work out the current through that from \$ I = \frac {V_{max} - V_{min} }{1k} \$.

Can you work out what value you require for R5 and R6 now so that the ends of the pot are held at the required voltages?