Can anyone give me an idea of how I can offer a variable that is dependent on the variable entered in a previous box? An example would explain it better:

Say I have a shed that comes as 8 feet, 10 feet, or 12 feet in length. A variable allows the customer to choose from those three options, with different prices set for each. Once he's selected one of those, now I want him to select from a second variable, this one to choose either one or two internal partitions for his shed. The trouble is, the COST of those variables alter dependent on what size shed he's chosen! For an 8 foot shed, he can have one partition for £50, or two for £100. But if he's chosen a 10 feet shed, one partition costs £75 and two partitions £150!

At least in theory, I could offer the variable to say:
8 foot shed, one partition £50
10 foot shed, one partiion £75, etc etc, but that allows customer to confuse the issue by selecting one size but a different size partition. What I'd really like is for the prices in variable box two to be dependent on the results in variable box one. The only thing I've thought of so far is some form of javascript page refresh, but I know even less about javascript than I do about zencart!!

Any bright ideas?