Creating a calculation in another form on User Input in current form

Creating a calculation in another form on User Input in current form

I have a form for Assignments based on a project lookup from a Projects Form
There can be several assignments per project and hey're delineated by Freelancer name.

When I update the final length of a project in the Project Form I want the Length_of_Project  field in the Assignment form for each person to update with that value.

How can I do this?
I've tried:
onUpdate and onInput for the final length field in Projects to Fetch Records and the For Each Record, change the value of the Length_of_Project filed in the Assignment form. It doesn't report errors, but no value gets entered in there when I update the length in the Project Form.

help?