Stadimeter question

When using the Stadimeter, is it necessary to line up the water line of my target with the center line of the periscope? I ask because after viewing the posts here on subsim, and watching many Youtube videos I've gotten confused. Half of them line up with the water line of target, and some do not. It seems like they all appear to get the correct data as they are successful in sinking their target. I've had limited results with Stadimeter, and have been using Sonar to acquire my range.

For example:
Stickied threads above have a few guides.
http://www.subsim.com/radioroom/showthread.php?t=111998 - This guide lines up water line with center of periscope.

http://www.subsim.com/radioroom/show...=108689&page=9 - Video posted here, he doesn't line up the water line.

There is numerous cases of this. I just want some clarification


Thank you for taking the time to read my post, and for any info/advice.

Silent Hunter 4 v1.5
Real Fleet Boat 2.0