7d camera 430ex11 flash

I'm actually experimenting with this myself.  I'm a total novice and still learning, so forgive me for anything silly I might say ...

But if you press and hold the Mode button for 2 seconds, the 430EX will go into Manual when it's a slave to the popup.

Correct me if I'm wrong.

The confusing thing for me is, when it's a slave, you can see the distance scale you can see when it's on the shoe.  So it's kind of hard to know what power you want to use, other than trial-and-error (or I suppose by taking a reading with the flash on the camera and then removing it?).

-ira