You have to set it at idle, but because you have digi-1 Im not sure what RPM would be proper. As the RPMS increase your ignition system also advance the timing. So settin the timing at improper RPM can cause you to set the timing incorrectly.
For a stock 16v the timing is to be set to 6* advanced at I think around 950 RPMS when idling. Its in the bentley, I cant recall the exact RPM.
For Digi-1 I would assume you would have to set the timing according to a regular digi-1 car because the ignition maps are different. A good place to start would be to find a corrado manual and look it up in there. However, because its a different motor you may need to adjust the timing settings accordingly and I have no clue what those may be... but I am sure someone on here will come on who does!
Typically all ignition timing is adjusted at idle, and you check for proper advance at higher RPMS such as 2500.