fuel pressure at full throttle

Pressure is a product of resistance to fuel flow. Stop and think of it, when the needle/seat is shut what happens? The pressure shows the highest. When an engine is idling it requires very little fuel so the needle/seat is pretty well shut and pressure shows high. At WOT the needle/seat is open most of the way because the engine is consuming a lot of fuel and because the needle/seat is open the pressure will be lower. If your engine isn't running out of fuel at high rpm in high gear you have no problem. I do agree with Bomber to set it at 7psi at idle, but just know that it'll drop when you hammer on it and that's normal.