The useful range of Rt is approximately 1k to 240k. The capacitor Ct is limited only by stray capacitances for low values and by leakages for large values.
The timing capacitor is reset (discharged) when the IN pin (Pin 1) is below the VOL(MIN) threshold. The capacitor reset time at the end of each controller pulse is determined by the supply
voltage and the timing capacitor value. The IC resets the capacitor to an initial voltage (VBE) by discharging it with a current of approximately 15 mA. Thus, a 0.1 µF cap is reset in approximately 25 µs.
i would go for 1k and 0.1uF. Then i would check if the timing is alright. If not i would increase Rt or Ct. Depends of the timing result i want to have.