You're welcome.

Yes, the WWII General Electric ESM/1 & ESM/2 "cross in glass" mirrors using the rearsight method are quite accurate, but there is more coordination involved. US Coast Guard testing found that subjects hit the plane more than twice as often with retroreflective aimers that with the "cross-in-glass" style when bobbing in a small liferaft/dinghy. I expect the difference would reduce on solid ground.

Here's a few links along those lines:
Illustrated instructions for aiming double-faced signal mirrors
Specific Instructions for GE "cross-in-glass" mirrors
Four examples of the ESM/1 4"x5" "cross-in-glass" mirrors
Two examples of the ESM/2 3"x5" "cross-in-glass" mirrors

Yes, trying to use a signal mirror for the first time when you need it is not a great idea.

The US Coast Guard, which requires all personnel to carry signal mirrors when afloat,
makes a point of testing their members on that skill.

Here's a nicely illustrated USCG PowerPoint signal mirror training package
http://www.completepqs.com/assets/files/bcm/ppt/BCM-02-09.ppt
It's on this USCG training site, with lots of related topics:
http://www.completepqs.com/index.php?id=57