The specified Center (+0.0) in F5
key is identical with the upper
value in red frame on the center
value of meter bar.
The specified Span Scale (+6.000)
in F6 key indicates the whole scale
of the meter bar, which means
+6.000 will be evenly divided into 2
ends of the meter bar that results in
-3.000 in the left end and +3.000 in
the right end as the figure shown.