Need help with Lookup Criteria

I have 2 forms: Project and Workarea

The Project form has a field- NAME; the Workarea form has a field- AREA and a field STATUS

I have a 3rd form, Workflow. On it I want a user to associate a workflow with a workarea. To do this they must choose a project name from the dropdown lookup. This works fine.

They then need to choose a workarea name (aka AREA) from the dropdown lookup. Here's where I can't get the criteria right to choose only the workareas associated with that project.

Also- this has to work in a mobile device- so no scripting etc that may limit its use.

Thanks.