Update ios app

General information for the app: the app calculates the late payment costs and interest applicable under UK law
Kind of development: Customization of existing app
Num. of web pages/modules: 3
Description of every page/module: 1. Standard splash screen

2. Function screen with 3 buttons

A. calculator
B. Request email
C. visit site

Calculator has two screens.
Screen 1 has:
One text input for amount of invoice
two date inputs, one for date due and one for todays date.

When data entered calculation is completed and displayed on a new screen. No user input.
Description of requirements/features: The calculator calculates the interest payable on any overdue invoice according to stipulated base rate.

Currently base rate hard coded into the app, new version ideally needs to take data from a web service/static file.

Source code for the existing app is available.
Extra notes: