Make sure you create the shared parameter FIRST!
Then use it to create a parameter in the family AND the project via Settings>Project Parameters.
In the family set the parameter to be equal to the parameter "W", an existing parameter, put it in the formula location next to your shared parameter. This way it will automatically fill in the weight per linear foot value in the family.
Then create a calculated value in the schedule that would read:
(Length/1')*Weight