Add the below method (with your custom field replacing "new_hoursfield") to the form onload and Total Cases/Minutes OnChange events.
function PopulateHours()
{
try
{
var hours = Xrm.Page.getAttribute("totalallotments").getValue();
if (hours==null) {
Xrm.Page.getAttribute("new_hoursfield").setValue(0);
}else{
Xrm.Page.getAttribute("new_hoursfield").setValue(hours/60);
}
}catch(err){
alert(err.message)
}
}