Shopify Coding - Custom length - Pricing Rules

General information for the website: We will be selling custom length drains.
Kind of development: Customization of existing website
Description of requirements/features: Greetings,

We need you to code some custom pricing rules into our shopify store.
When the customer types in a custom length in a text field we need the price to be calculated from it's value.

Example:
Price of Drain up to 1000mm = $320
Additional price above 1000mm = $12 per 100mm

if 100-1000 is entered then the price would be $320
if 1200 is entered then the price would show $344.

We don't mind weather the price is calculated in brackets of 100mm or exactly, Which ever is easiest.
So if the customer enters 1244 the price would be as if they typed 1300 which would be $356.

Integrated with https://apps.shopify.com/advanced-custom-products.

Let me know if I've left anything out.
Extra notes: Extra Information:
Shopify Basic
Debut Theme