FormCalc won't calc!

I have created a subform with repeating rows and I'm trying to have a calculated field keep track of the cumulative mileage.

The structure is

form1
sectionB
header
detail
button1
date_from
date_to
mileage
delete
total
total

In total I have amended the script from the Purchase Order template but I must be doing something wrong as it isn't working. I have applied the script below to the calculate event on the total field.

//Verify at least one instance of the mileage field exists.
 
if (exists(details[0].mileage) == 1) then
  Sum(detail[*].mileage)
endif