I'd use a spreadsheet for the fret spacing. I'd probably use perl where I might have used BASIC. eg.

  #! usr/bin/perl
  print "Scale length ";
  $sl = <>;
  $nl = $sl;
  print "fret\t from prev fret\t\t new scale length\t from nut\n";
  for ($fret=1; $fret<=12; $fret++){
          $prev = $nl / 17.817153;
          $nl = $nl - $prev;
          $dn = $sl - $nl;
          $out = sprintf "%5d\t %7.3f\t\t %7.3f\t\t %7.3f", $fret, $prev, $nl, $dn;
          print "$out\n";
  }

  jon@worthy:~> perl scalelength.pl
  Scale length 650
  fret     from prev fret          new scale length        from nut
      1     36.482                 613.518                  36.482
      2     34.434                 579.084                  70.916
      3     32.501                 546.583                 103.417
      4     30.677                 515.905                 134.095
      5     28.956                 486.950                 163.050
      6     27.330                 459.619                 190.381
      7     25.796                 433.823                 216.177
      8     24.349                 409.474                 240.526
      9     22.982                 386.492                 263.508
     10     21.692                 364.800                 285.200
     11     20.475                 344.325                 305.675
     12     19.326                 325.000                 325.000
  jon@worthy:~>