X is only for sticker upgrades from Coach to First and will not show up in the inventory (available or not) until 100 hours prior to departure. There really is no fool-proof way of trying to maximiize your chances. If it is a two-class flight, look at A inventory which is the mileage award and discounted first inventory. If it is still available the closer you get to the 100 hour window, the better the chances will be for an X upgrade. But, this is just a guide and by no means gospel. I just saw the opposite happen on a flight from HNL-ORD where on the day of departure there was plenty of X and no A inventory. Same seats in FC; you could use stickers to upgrade but not miles - go figure.