I'm selling paint, selling it per litre.

I sell 2,5, 5 and 10 litre cans.

My prices are per litre, ie:
2.5litre: $5 per litre
5 litre: $4.5 per litre
etc.

What I want on my product page:
A dropdown with the option to choose a can (2.5/5/10 litre)
So I selected Price by Attribute and I maked 3 attributes:
2.5
5
10

The 2.5 attribute has the following options:
Price: $5 (because it's $5 per litre)
Weight: 2.5
Price Factor: 2.5 (because it's 2.5 litre)

But, Zen calculates it wrong... I don't get $12.50 but something else (Tax included)

What am I doing wrong?

Here's a link:
LINK
additional info is on the product page