Get data from Sub Form to Main Form

Get data from Sub Form to Main Form

Hi guys, I have been reading around a lot and i cannot seem to get a "Total" value of all sub form entries per customer.

The Main form is called Customers and has a field called "Total Cover Amount".
THe Subform is called "Appliance Under Cover" and the sub form has a field called "Cover Amount" and there can be multiple appliances per customer of varying Cover Amounts. 

I just want the "Total Cover Amount" to be the sum of all "Cover Amount" but I can't figure out whether I can do it with a formula or I have to use a script as I am trying to get the data from a subform. 

Can anyone help? I am new to Zoho and Deluge but any help would be massively appreciated! I have been reading other forum posts on similar topics but have not had any luck at all at getting closer to making this happen :(

- Thanks in Advance