As a general rule, I like to position my chest centered to the 15th fret of most steel guitars. One usually plays most of the music from fret 0 to 17th fret. So positioning yourself in the 15th fret area, keeps your site and left arm position comfortable for playing. If you play above the 17th fret, there will be a certain amount of the left arm crossing over your center point, but is not the majority of your playing, and won't be as uncomfortable as if you were to sit more towards the 12th fret. If you were to sit too far to the right of 15th fret (for right handers that is), your eyesight for the fret line references would be harder to see and adjust for good quick intonation.