sum across fields

Hi
i have these 2 forms to manage time spent with customers
one is customer details and the other is time spent with each one
the first form has a name field and a amount of hours pre-purchased
the second form has a start date field(timestamp), a end date field(also a timestamp) , a lookup to customer field on the other form and a formula field that gives me the total time per record in second form

form1 - customer details
field NAME
field TIME PURCHASED
field HOURS REMAINING

form2 - appointements
field "START DATE"
field "END DATE"
field lookup "NAME FROM FORM!"
field formula TIME (end date minus start date) and it shows me time in hours

i need to sum the values for each record in form2 and copy the value to HOURS REMAINING in form1

thanks for your help