Something like:

use strict;
use warnings;

my $kDone  = 0.95;
my $kPivot = 100;
my $kDelta = 0.2;

for my $pair (
    [10000, 9500], [1400,  1389], [100, 90],
    [100,   80], [10, 8], [10, 7], [5, 4], [2, 1], [1, 0]
    )
{
    my ($toDo, $done) = @$pair;
    next if !$toDo;

    my $ratio = $done / $toDo;
    my $adjusted = $kDone - ($kDelta - $toDo / ($kPivot + $toDo) * $kD
+elta);

    printf "%5d %5d %5.2f %5.2f %s\n", $toDo, $done, $adjusted, $ratio
+,
        $ratio > $adjusted ? 'Done' : 'Working';
}
[download]

Prints:

10000  9500  0.95  0.95 Done
 1400  1389  0.94  0.99 Done
  100    90  0.85  0.90 Done
  100    80  0.85  0.80 Working
   10     8  0.77  0.80 Done
   10     7  0.77  0.70 Working
    5     4  0.76  0.80 Done
    2     1  0.75  0.50 Working
    1     0  0.75  0.00 Working
[download]