/* RMP Reference Tables */
$T = array(
  /* Table 26: Rural, D stability, 3.0 m/s [(lbs/min)/(mg/L), mile] */
  '26' => array(
        28, 0.1,     40, 0.1,     60, 0.1,    220, 0.2,    530, 0.4,    860, 0.4,
      1300, 0.5,   1700, 0.6,   2200, 0.7,   2700, 0.8,   3300, 0.9,   3900, 1.0,
      4500, 1.1,   5200, 1.2,   5800, 1.3,   6800, 1.4,   8200, 1.6,   9700, 1.8,
     11000, 2.0,  13000, 2.2
  ),
  /* Table 27: Urban, D stability, 3.0 m/s [(lbs/min)/(mg/L), mile] */
  '27' => array(
        68, 0.1,    100, 0.1,    150, 0.1,    710, 0.2,   1500, 0.3,   2600, 0.4,
      4000, 0.5,   5500, 0.6,   7300, 0.7,   9200, 0.8,  11000, 0.9,  14000, 1.0,
     18000, 1.2,  26000, 1.4,  31000, 1.6,  38000, 1.8
  ),
);

/* Calculate LFL in mg/L */
$lfl = [LFL:%v]*[MW:g/mol]/2.45;

/* Calculate release rate/endpoint ratio */
$r = [qgr:lb/min]/$lfl;

/* Get reference table */
$t = $T[[RMP_RT]];

/* Search for endpoint distance */
for ($i = 0, $n = count($t); $i < $n; $i += 2) {
  if ($r <= $t[$i]) break;
}
$d = $i >= $n ? '*' : $t[$i+1];
if ($d == '.') return  '<0.1';
if ($d == '*') return '>25.0';
return $d;

See U.S. EPA (1999), Section 10.1 - Vapor Cloud Fires.