Creating an indesign fillable form with computation (addition)

I am trying to set up a bespoke menu order form using Indesign CC interactive forms. I have my master template set up however I cannot quite get how to do the following for the viewer to be able to calculate an amount across a row and have an automatic table in the final costs box.

E.g

Breakfast at £13.80

Then boxes across covering hours in the day where the reader can allocate different quantities of said breakfast until the amount is automatically tallied at the end.

For example

Breakfast at £13.80 x2 x3 x4 x2 to a total of £151.80.

I understand I would be looking at a custom script in Acrobat Pro as probably the best way to do this but cannot understand how.

Any help would be so gratefully recieved.

Kindest

Lee