Yes, you need to alter your design. Also, you appear to be treating "layout" and "table" as the same thing in your description. Be aware that while you may have a table on the tables tab in manage | database, a "box" on the relationships tab and a layout all with the same name, they are three different objects with different roles to play in your solution.
Typically, estimates, invoices and purchase order all follow the same basic data model in relational databases such as FileMaker. The only difference is in the name of the tables.
Estimate-----<LineItems>-------Products (and/or Services)
Estimate::__pkEstimateID = LineItems::_fkEstimateID
Products::__pkProductID = LineItems::_fkProductID
The last table may not exist if the products or services are customized to the point of being unique to each estimate.
A common way to set up your layouts is to set up a data entry layout for creating estimates where you base the layout on Estimate, but include a portal to LineItems for listing the details that make up that estimate. Calculation fields in the line items table can compute a line item cost (unit price times quantity) and other calculation fields in Estimates can compute a total estimate cost.
For printing purposes in FileMaker, we often turn this around a bit and set up a list view layout based on LineItems that includes fields from Estimate placed in Header, footer and/or grand summary layout parts. You pull up a found set of all line items for a given estimate (there are several ways to do this), sort it the way you want and you are good to print or save as PDF. This works better than printing from the data entry layout with a portal as you can print out estimates that are more than a page in length and which don't have a lot of "wasted empty space" for not needed additional line items.
I've probably used a number of terms that are new to you here. Please look them up in Help, so some researching and feel free to ask for clarification here.