I have a couple of short-cuts that I sometimes use (still needs a lot of test-renders,m but not so many).

1. Position/angle the mirror from the top view. Remember that in order to see the reflection, the angle between the mirror and the camera must be the same (in the opposite direction) as that between the object and the mirror. Of course, this gets much more tricky if your mirror and/or camera is rotated in any angle other than the Y axis..

2. Try using a (very) big 1-sided square for the mirror during set-up. That way you can (almost) always see exactly where the reflection is. Once you've got the reflection positioned where you want it, replace the big square with the mirror, using the same angles (or shring the square down to the correct proportions for the mirror and delete the reflecting surface from whatever prop you're using for the mirror - only feasible if the mirror is rectangular..)

Hope that helps..

Cheers,
Diolma