$percentage = 13;
$desired_net = 100;
$charge = sprintf "%.2f", $desired_net / (1 - $percentage/100);
$charge = sprintf "%.2f", $charge + .01
   if $charge * (1-$percentage/100) +.5 < $desired_net;
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(Update: added +.5, assuming reasonable rounding on the part of the site)