Here's how I figured out to use my existing analog clutch pedal and a button on my steering wheel to work as a double clutch system for launching the Formula 3 in iRacing.
Map a button to the "Second Clutch" input in iRacing. Adjust the CalibMax value for your analog clutch in the joyCalib.yaml file to give the range headroom above the device maximum. Find a value that puts the max analog pedal right at the bite point. Launch the car when the lights go green by holding full throttle in 1st gear and release the button - then gently release the pedal as you get going.
