7M-GE, not GTE, Jeeves... Isn't it a different goose? I'm not sure on the base ignition timing for the 7M-GE (the 5M's is 10 degrees BTDC I reckon, don't quote me on it), but what you can do is run premium fuel and do an adjustment as you go: advance, and do some hard, full-throttle runs: if you hear excessive knock (detonation), you know to retard it a bit. Keep at it until you've reached the right level. Some slight spark knock under load is normal.
Do keep in mind that if you're in the colder months, you're going to have to retard timing when summer hits - detonation is more likely as the temperature goes up.