Err,
Is the Flight ID the aircraft call sign "G-XXXX"?
Group ID left blank.
In fact for PilotAware/P3I it is any ASCII printable string so yes, "G-XXXX" is fine.
However, for a Mode-S transponder the FlightID is more restrictive, for example "-" is an illegal character, so for a transponder you would probably set FlightID to be "GXXXX"
So just to summarise, I think "G-XXXX" is a good choice, of course for some users they will not have a tail reg, eg hang glider, drone etc.
The FlightID is the text that will be displayed in other PilotAware systems when they see your A/C, if you do not enter a Flight ID, then the Hex-ID is displayed.
I hope that makes sense ?
Thx
Lee