Local/Global Scaling? I Have this line right in the cleavage of a character that I'm modeling, and I am using a mirror modifier to create her. 

I was given a tip to reset the "middle" circle of vertices (The ones that are the closest to the mirror/Y Axis) by pressing "S" then "Y", "Zero" and then "Enter", but when I do this, it resets the entire action, because the "Local" "Zero" is where I started scaling from, whereas I want it to move to the "Global" "Zero".

Her chest
Shift + S
Cursor to Center (alternative to 1+2: Shift + C)
. to change to Pivot Cursor
Scale your thing