That's not what I said.

I said that I used the fault code reader to set the engine into diagnostic mode. It can't actually change the timing.

Once the engine is in diagnostic mode the timing is set to a static 10 degrees regardless of RPM or anything else. You can then use the timing gun to check that it is set correctly. If the timing gun shows the timing at anything other than 10 degrees something is wrong.

The timing on a proper FRST with the flywheel CPS sensor is not adjustable unless you use the octane ground cable and that only retards it by 2 degrees IIRC. However on a car that has been converted to EFi using a aftermarket trigger wheel then it often is adjustable. You then need to set the timing to be 10 degrees in diagnostic mode in which case it will be spot-on.

I'm sorry if I didn't explain this very well before - I wrote the reply in a hurry.