I haven't worked with Lawena in a while (Premiere is the bane of my editing right now), but when I have, I always made sure to leave at least 500 ticks of space before and after the bit I actually want.
My process is fairly simple, but could be considered tedious:
Reviewing your ticks, whether sent by you or straight from your killstreaks.txt: I load up the demo, and go maybe a thousand or so ticks backward from my given bookmark, or just start at the tick. Usually the former, but I've done both.
I may review one small section of a demo pertaining to that set of ticks multiple times, then I right down a starting and end point for that demo. I've done this both in my killstreaks.txt for my own demos, on paper, and in other files depending on the project. These starting and end ticks are what go into the VDM generator.
It end up looking something like this: http://puu.sh/jaGa1/bd93ac3dcd.png (This was for my GXL 2014 HL demos)
As for your later post; sometimes I find I'll go through bookmarks, and there's nothing interesting, before or after the marked number. At that point, I label the bookmark as per the RuneScape game message "nothing interesting happens." It happens. Just label the tick as useless, and move on.
In short, always allow lots of space before and after your cut when recording demos through Lawena. It's often very helpful to keep track of your bookmarks: label them, note when or when you haven't recorded them, and make sure you know your starting and ending points.
Hope this helps, happy editing!