set numbers [list 625000.0 520.0 -1110.0 42.0]
 
proc fix_sci {n} {
  if {![string match "*e-*" $n]} {return $n} ;# Not a sci-fmt number
 
  set a 9
  set b 12
 
  regexp -- {(-?[0-9.]+)e-0*([0-9]+)} $n - front exp
 
  if {$exp <= $a} {
    set dif [expr {$exp-$a}]
    set front [expr {$front/(10.0**$dif)}]
    set exp $a
  } else {
    set dif [expr {$exp-$b}]
    set front [expr {$front/(10.0**$dif)}]
    set exp $b
  }
 
  return [format %3ge-%.2d $front $exp]  
}
 
puts [format "%15s%15s" "Not formatted" "Formatted"]
puts "---------------+--------------"
 
foreach n $numbers {
  set fmt [fix_sci [format %3.6g [expr {$n * 1e-12}]]]
  set nfmt [format %3.6g [expr {$n * 1e-12}]]
  puts [format "%15s%15s" $nfmt $fmt]
}

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