I'm not quite sure I follow you, but this is what I understand (correct me if I'm wrong):
IV: Location start point (fixed/variable)
DV: difference between end point (as presented) and location where subjects actually end up, in inches
Did I summarize your DV correctly? You said you measure end point, but since that seems to be controlled by the software I don't see how that can be your DV. So is your DV instead the difference (inaccuracy) between target end point and subject end point?
And your question is how start point being fixed/variable affects this difference (or rather, the variability, since the mean will presumably be 0 if there's no bias towards either side)?
That's not really a stats question I think. It's rather the topic of your research, so it's something you can empirically determine and/or find in the literature.