Manufacturing Cost calculator; is what I am trying to do feasible?

The drop down lists for the materials when add8ng a materials estimate lin3 item will be from that materials table itself. The whole list of materials will be your drop down list.

There should not be a relationship between estimates and materials. Delete that relationship if it exists.

When you go to add a line item to the estimate you are not creating a new Master record fir a new material. You are choosing an existing material when you add the material estimate line item.