Littlefuse's Phase Control Using Thyristors is an excellent reference on the subject of both thyristors and triacs.
Littlefuse's application note suggests two 100 Ω with a filtering capacitor.
You can omit C1 but may find the circuit more prone to triggering by noise. For 230 V operation double the resistor values.
You also need to calculate and include Rin. Typically you should aim for 10 mA through the infrared opto-LED.
Figure 2. The LED loadline chart. Source: Loadline resistance calculation. (Disclaimer.)
For 10 mA in the infrared opto-LED, slide across from 10 on the If axis to the IR LED curve and pick the closed loadline: 390 Ω in this case. That's your Rin value.
Other links:
- Thyristor Theory and Design Considerations Handbook by ON Semiconductor.