I am creating a very simple calculator for a web page. basically it is (entered value - entered value * 4) and (entered value + entered value * 4).
the first (subtraction) works, but the second (addition) does not format properly and returnes both values as strings joined together (ex. 1.2340.525).

This function works:
function externalmax(){

if (maxafterpitch.value=="") maxafterpitch.value="0";

if (maxthickness.value=="") maxthickness.value="0";

var pinum=(maxafterpitch.value-(maxthickness.value*4))

show("The Maximum Suggested PrePlate Pitch Diameter Is:<br><input type=text size=20 value="+pinum+">. ");

}


this function does not work:
function internalmin(){

if (mindiameter2.value=="") mindiameter2.value="0";

if (maxthickness2.value=="") maxthickness2.value="0";

var pinum=((maxthickness2.value*4)+mindiameter2.value);

show("The Minimum Suggested PrePlate Pitch Diameter is:<br><input type=text size=20 value="+pinum+">. ");

}
(returns: 0.002.02500)

Is there a method to format math values returned in javascript?
Thanks for any help.
MP