The format specifier %8.6f says place the number in an 8 char wide space and include 6 digits of decimals. One of the two extra spaces is occupied by the decimal point; the last is used for the integer part of the value. Which in this case will be restricted to a single digit before the field width will be overridden.

A specifier of %6.6f doesn;t make much sense because it leaves no room for the decimal point or integer part of the number.

For the most flexible representation, use %12.6g (or larger) which will make a reasonable fist of displaying a fairly wide range of values in a semi consistant fashion:

[0] Perl> printf "%12.6g\n", $_ for map{ "1.23456e$_" } -12 .. +12;;
1.23456e-012
1.23456e-011
1.23456e-010
1.23456e-009
1.23456e-008
1.23456e-007
1.23456e-006
1.23456e-005
 0.000123456
  0.00123456
   0.0123456
    0.123456
     1.23456
     12.3456
     123.456
     1234.56
     12345.6
      123456
1.23456e+006
1.23456e+007
1.23456e+008
1.23456e+009
1.23456e+010
1.23456e+011
1.23456e+012
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The floating point formatting by sprintf has always -- going way back to the earliest of C compilers -- left a lot to be desired.

You might also find the threads at Engineering FP notation & sprintf and Display floating point numbers in compact, fixed-width format of interest.