some people make a mark on their forehead and place the piece just behind it.....then remove the mark after the piece is on.
If you are new to this , and only overlapping your own hair on one side by 1/4 inch, you are doing very well.!! there is a learning curve to this, the more you do it, the more accurate you will become. if your piece is the right size, and it sounds like it is, you could try applying one side first and then laying it down side to side, if one side lines up perfectly , then the other side should fall in the right place....unless you are stretching it a bit too much.
DebbieS.