"Official" method.

- Accelerate up to 4th gear
- Pedal to the floor at 1500 rpm
- hit the brake at 3500 rpm, keep the throttle fully depressed though (not longer than 5 secs)
- read your gauge now
- let your brakes cool down

On my car (std actuator at the moment) it makes a huge difference, whether I'm in first, second, third or fourth gear. Don't know what it's like with an uprated actuator, though