There's a handy chart for determining the effect of elevation on power:



At 12,000 feet you should be producing 74 to 78% of the power you could produce at sea level, or between 80 to 83% of what you produce at home at 4500 feet.

(note from the chart the reason you feel strong at 4500 feet, moving from 7000 feet is that your FTP went up about 5 %).

The take away from all this is that your power is going to be down somewhere around 20% at 12,000 feet, even if you're well acclimated. In fact the pure effect of the decrease in oxygen is much stronger than the difference between acclimated and unacclimated)

So all you can do is know your power is going to be well done, and train to raise your FTP as much as possible before, and pace yourself on the ride knowing your 20% weaker.