Setting position using Trial Levels

When using Trial levels to set the positions of a list of stimuli what does SuperLab use as the reference point on the stimulus? For example, I have several words of different lengths that I want to present simultaneously in a vertical orientation. I set the positions to be -200 and +200. When I present the words at the +200 location I expected SuperLab to use the left edge of the words but it doesn’t seem to. The groups of words appear in slightly different locations over the course of the experiment. When I use the -200 position I expected SuperLab to use the right edge of the longest word, but again this doesn’t seem to be the case.

Could someone clarify what gets positioned at the location specified. Thanks.

This is easily set in the Event Editor’s Stimulus tab. In that tab, click on the Settings button; a dialog appears. Then click on the Position tab. You can change the reference point by clicking on one of the circled options.