I've been looking at zen cart for a few days now and I really like it. I'm sure I can get it working to suit my needs however I've come across a big hurdle.

I need to add 2 attributes which both affect the pricing by a factor and I can't manage it.

Both attributes are dropdown boxes numbered 1 to 5, and both set to 1 by default. The first attribute will double the price if you select 2, three times the price for selecting 3 etc. It's not a quantity but more of a size attribute.
The second attribute is similar, but instead of adding the 100% of the original price each time, I need it to add 75% (100%, 175%, 250%, 325%, 400% etc)

I can get both working independent of each other, but not together. For example:
I select 2 from the first box and then 2 from the second. Original price is £10.
The first box should double the price and the second should add 75% (of the already doubled price from the first box). Total price is now £35
My set up gives £27.50 (first box adds double the original price - £10 and the second box adds 75% of the original price)

I realise this is complex and will clarify if I have not made this clear.

Any help would really be appreciated.