I am setting up a Magento CE store for a friend and have a question regarding product variations.

He sells awnings and sun-blinders to people and have a product with the following specifications:

There are 3 depths for an awning: 2 meters, 3 meters and 4 meters
And multiple sizes in widths, but prices are pr. 50 cm and he want the customer to be able to write the exact measurement in the page. 

Example: 2 meter depths and 3,55 m width is calculated down from: 2 meter depths and 4 meter widths. The product is simply cut down from the next possible width size. 

I really hope any of you have an idea how to code this up or maybe knows an extension that deals with these type of products.

Thank you in advance