how do you know where that is on the neck ?
Depends on where the Root note/tonic of the scale you want is on the neck.
The lowest note in Box 1 (6th string, right facing) is a minor Pentatonic root and the upper note (6th string, left facing) is a Major Pentatonic root.
Box 3 has the minor root on the 5th string, upper note (left facing) and the Major root on the 4th string, lower note (right facing).
The boxes are moveable and will be in different places in different keys. (But always connected.)
Line the roots of these boxes up with the root of the minor or Major pentatonic you want to play.
EXAMPLES:
Am Box 1: Your first finger will be on A at the 6th string,5th fret.
C Major Box 1: Your pinky will be on the C at the 6th string, 8th fret.
Am Box 3: Your 3rd finger will be on the A on the 5th string 12th fret.
C major Box 3: Your 1st finger will be on C on the 4th string 10th fret.