Sorry we all seem to be ignoring this, I think maybe noone's exactly sure what you mean.
I can picture the cue, do you just have to estimate and mark it or do you have to calculate how far along it would be?
If it's just marking it, all I would do is place a dot just where you say and label it as centre of gravity. Sometimes you have to show the forces acting, in which case I'd also draw the W=mg arrow vertically down from that point.
Does that help?
Sorry, I'm guessing a bit about what you've actually been asked to do here.