Yea, a 12x6 on most 480 motors is pushing your luck. Get a watt meter. If you going to fly electric and play with diffrant props, it is a MUST have item. This is the one I use. $24

http://www.hobbypartz.com/26p-twometer.html

I hear a lot of people wanting to change the prop and get longer run times with the same performance. Doesn't really work that way, you will always have to give up something, one way or the other. Going down in prop size will give you longer run times, but will cost a little in climbing power. The only way to get longer run times with the same prop is the get a bigger battery.