God its been so loooong since I have even thought about this, BUT, I think its 10 degrees, I would wait for others to chime in tho, I'm getting old and my memory sometimes fails me. LOL
EDIT I did a little research., Ignition Timing
Most 5.0 Mustangs are delivered from the factory with the ignition timing set from 4-6° BTDC (Before Top Dead Center). Stock its set at 10 degrees, if you are running premium fuel you can start at 13°-14°. If you have no detonation, increase upward. Timing is free horsepower and will run better up to 19°-21° of timing. This is some info I found, I cant remember what I have my 92 LX set at, so I do not know if this is accurate.
Best of luck!!