Muchas Gracias Caricatos funciono a la perfección; para quin le interese aqui pongo el codigo que utilicé:
Código PHP:
<script language="JavaScript" type="text/javascript">
<!--
//Relacionamos la seleccion de los menus a una variable.
function alcambiar(){
with(document.formulario){
cp_1 = 1;
cp_1 += eval(select1.options[select1.selectedIndex].value);
cp_2 = 0;
cp_2 += eval(select2.options[select2.selectedIndex].value);
if(cp_1<=cp_2){
alert("Primero debe seleccionar la fecha mas reciente");
}
}
}
conprorroga = new Array('0', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '3.00', '3.00', '3.00', '3.00', '3.00', '3.00', '3.00', '3.00', '3.00', '3.00', '3.00', '3.00', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '3.50', '8.25', '8.25', '8.25', '8.25', '8.25', '8.25', '8.25', '8.25', '8.25', '8.25', '8.25', '8.25', '8.25', '8.25', '8.25', '8.25', '8.25', '6.56', '6.56', '5.07', '4.41', '4.32', '4.32', '4.34', '4.50', '4.93', '5.35', '5.27', '5.12', '5.08', '5.32', '5.63', '5.58', '4.13', '3.82', '4.05', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '1.50', '1.50', '1.50', '1.50', '1.50', '1.50', '1.50', '1.50', '1.50', '1.50', '1.50', '1.50', '1.00', '1.00', '1.00', '1.00', '1.00', '1.00', '1.00', '1.00', '1.00', '1.00', '1.00', '1.00', '1.80', '1.40', '1.50', '1.80', '1.80', '1.80', '1.70', '1.50', '1.60', '1.50', '1.2', '1.6', '2.00', '1.07', '0.89', '1.13', '1.50', '1.91', '1.93', '2.00', '2.00', '1.87', '1.56', '1.81', '2.00', '2.00', '2.00', '2.00', '2.00', '0.57', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '1.45', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '2.00', '0.48', '0.56', '1.08', '1.75', '1.92', '1.67', '1.93', '1.69', '1.69', '1.29', '1.66', '1.65', '1.16', '0.29', '0.81', '1.54', '1.63', '1.69', '1.50', '1.77', '2.00', '2.00', '2.00', '2.00', '2.00', '1.95', '2.00', '2.00', '1.90', '2.00', '2.00', '2.00', '2.00', '1.80', '1.94', '1.49', '1.34', '0.98', '1.38', '1.43', '1.33', '1.62', '1.59', '1.54', '1.53', '1.37', '1.48', '1.48', '1.22', '1.77', '2.00', '1.53', '1.54', '1.61', '1.40', '1.85', '0.85', '0.65', '1.08', '1.03', '1.19', '0.41', '1.48', '0.86', '0.69', '1.11', '0.88', '1.07', '0.92', '0.76', '0.95', '1.34', '1.43', '1.29', '1.38', '1.41', '1.31', '1.17', '1.07', '1.03', '1.01', '1.01', '0.99', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0.75', '0')
function calcular1(){
// Se extrae un nuevo Array en base al rango seleccionado en los menus.
sumatorio = conprorroga.slice(cp_2,cp_1);
sumatorio2 = sumatorio.reverse();
//Introduciomos el String "+" entre cada valor del nuevo array para luego evaluarlo con eval.
res = eval(sumatorio2.join("+"));
//Redondeamos el resultado de la operacion
var flot=parseFloat(res);
var resultadoRedondeo=Math.round(flot*10000)/10000 ;
//Imprimimos el resultado en la casilla correspondiente.
document.formulario.resultado1.value=resultadoRedondeo + "%";
}
</script>