You could do the math in the SQL or with php on the page. In the sql it would look like:
<?php
$WADADirResD_details = new WA_MySQLi_RS("WADADirResD_details",$ABANet,1);
$WADADirResD_details->setQuery("SELECT id, Division, Location, Date, Name, MemberID, BigBass, BigBassFee, Place, DQ, Day1_Fish, Day1_Alive, Day1_Short, Day1_Penalty, Day1_BigBass, Day1_ScaleWeight, Day1_TotalWeight, Points, Payout, Payout_Description, Day1_ScaleWeight - Day1_Penalty AS Day1_EffectiveWeight FROM DirResD WHERE id = ?");
$WADADirResD_details->bindParam("i", "".($_GET['id']) ."", "-1"); //Paramid
$WADADirResD_details->execute();
?>
And then you could display the value in the totalWeight field on the page with:
<?php echo($WADADirResD_details->getColumnVal("Day1_EffectiveWeight")); ?>
Or you could leave your Recordset SQL statement as it is and use this to display the value:
<?php echo($WADADirResD_details->getColumnVal("Day1_ScaleWeight") - $WADADirResD_details->getColumnVal("Day1_Penalty")); ?>