If you use trace bitmap does it usually give two lines for the outline of the character being traced using brightness cutoff? When I try centerline tracing it gives one line but the tracing isn't very accurate even at error threshold at 1. I've seen another post of similar topic but I want to know is there any other way other than centerline tracing and how do you optimize tracing of a single line of the stroke of a character. 

What I mean by two lines is a fill with a stroke on either side of the fill. See attachment for example. 

If you want a dynamically stroke maintained an outline trace (like fonts are outlined shapes for instance) is recommended - Centerline tracing works maybe best on CAD based drawings and are always problematic at cross sections, because Bézier curves don´t know anything about it. In essence it´s not 2 lines it´s a colour filled shape to be precise.

So I´m not sure I see anything wrong here.

Thanks