timesheet calculations

I know this topic has been covered extensively but I have spent too much time searching past questions/answers.  If anyone can point me to the right place I would much appreciate it. 

I am creating a timesheet in PDF form and although I can calculate the difference in starttime, lunchout, lunchin, endtime the result does not format properly.

Instead of getting say 5.5 hrs the result is 5.30 hrs.   7:00 starttime; 10:30 lunchout; 11:00 lunchin; 13:00 endtime.

Here's my code so far:

// start
var start = this.getField("Start1_1").value;
var startArr = start.split(":");

//lunchout
var lunchout = this.getField("LunchOut1").value;
var lunchoutArr = lunchout.split(":");

//lunchin
var lunchin = this.getField("LunchIn1").value;
var lunchinArr = lunchin.split(":");

// finish
var finish = this.getField("End1_1").value;
var finishArr = finish.split(":");

// difference
var hourDiff = Math.abs(lunchoutArr[0] - startArr[0])+ Math.abs(finishArr[0]-lunchinArr[0]);
var minDiff = (Math.abs(lunchoutArr[1]-startArr[1])+ Math.abs(finishArr[1]-lunchinArr[1])/60*100);
if (minDiff.toString().length == 1)
    minDiff = '0' + minDiff;

var output = hourDiff + "." + minDiff;
event.value = output;

Thanks so much for any assistance.  I really need to get this finished.

Lynda