From your explanation I think you need "dependant" attributes and Zen Cart and any Opensource that I am aare of does not have support for this.

You might re-think this and make 1-? attributes along this line

base cable up to 1 ft

Attributes:
1-5 ft = $?
5-10 ft = $?
10-15 ft = $?
15-20 ft = $?
etc

add a entry box for and if inches is important

Enter the exact length