When to launch yourself into the stream on a roundabout ? On your final approach, look to your right for a car's :
1. Signal (of its direction)
2. Position (from the inside of the roundabout)
3. Speed (of approach)
4. Direction (of looking, by the driver)
5. Hand positions (on the wheel, by the driver)
... to decide if it's leaving the roundabout or staying on.