i set up a test page using a dynamic dropdown for another thread.
the result is a VERY simple example, but should give you an idea.
on choosing a start number from the first dropdown, the second list (end number) is limited, so that only numbers greater than the first number are available.
eg if you choose 200 from the first dropdown, you only have numbers above 300 from the second.
if you choose 500 from the first, you only have numbers above 600 from the second.
any more questions, please ask
A version of the sample in the help documentation can be found here dynamicselection.php