I just did a similar formula so I thought I would be a sport and post it here. it will produce the whole 12 digit UPC code. In this case my client was working off the Quick Base Record ID# after their own prefix.
var text UPCMissingCheckDigit = ToText(742797) & ToText([Record ID#]+84000);
// test a few kitchen items :)
//var text UPCTEST = "62815401171"; // Shreddies (Check digit should be a 2)
//var text UPCTEST = "06410027191"; // All Bran (Check digit should be a 3)
//var text UPCTEST = "05574250317"; // Sugar Cones (Check digit should be a 3)
var text String = $UPCMissingCheckDigit;
var number A = ToNumber(Mid($String,1,1));
var number B = ToNumber(Mid($String,2,1));
var number C = ToNumber(Mid($String,3,1));
var number D= ToNumber(Mid($String,4,1));
var number E = ToNumber(Mid($String,5,1));
var number F = ToNumber(Mid($String,6,1));
var number G = ToNumber(Mid($String,7,1));
var number H = ToNumber(Mid($String,8,1));
var number I = ToNumber(Mid($String,9,1));
var number J = ToNumber(Mid($String,10,1));
var number K = ToNumber(Mid($String,11,1));
var number Odd = ($A + $C + $E + $G + $I + $K) * 3;
var number Even =($B + $D + $F + $H + $J);
var number Sum = $Odd + $Even;
var number CheckDigit = 10-Rem($Sum,10);
$String & ToText($CheckDigit)
