How can I get 'lead point' for aircraft to intercept final course?
I was taught the following rule of thumb for providing a final vector to final, in the USA, using the STARS radar system which (when using ADS-B inputs) has a one-second update time:
Set the Predicted Track Line to 30 seconds. Vector the aircraft to a 90º base-to-final. When the PTL reaches the extended runway final, issue the Position-Turn-Altitude-Clearance. The aircraft should roll out on a good heading to intercept final approach course.
This is obviously at low altitudes and relatively low speeds, 230ish knots or below. Theoretically (because the method uses the aircraft's speed instead of a fixed distance) this method should work for any aircraft going any speed, but that may not always pan out. In particular, some military aircraft will be flying rather quickly on the base leg and slow down as soon as they get their dogleg turn-to-final, which could end up leaving them too far from final for a legal intercept.