Gravity Forms, This works

/** Gravity Forms Presubmission for Rounding */
add_action("gform_pre_submission_1", "pre_submission_handler");
function pre_submission_handler($shipAmt){
                                $min_due = 200;  //Minimum due amount
                                $inc_start = 25000; //Amount at which we add add'l dues
                                $inc_span = 5000;   //The span for each due increment
                                $inc_amnt  = 50;                                //The incremental addition to the dues
                                $dues = $min_due + (max(0, floor(($_POST["input_1"]-$inc_start) / $inc_span) + 1)) * $inc_amnt;
$_POST["input_1"] = $dues;
}