Estimates, Quotes, Orders, Invoices, Purchase Orders all record the same basic info even though they represent different things for your business. You need to record info identifying the client and/or vendor; you need to list each item purchased or sold, a cost for each item as well as a grand total that may also include additional charges such as tax and/or shipping.
Thus, you may want to look at the invoices starter solution that came with your copy of either FileMaker 11 or 12.
The table and field names will be different (and are different for the two starter solutions also) but the basic relationships will be this:
Estimates::__pkEstimateID = LineItems::_fkEstimateID
ProductsServices::__pkProductID = LineItems::_fkProductID
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained
You then put a portal to LIneITems on your Estimates layout and create one new related record in the portal for each size tank that is part of your estimate. Qty, Size, unit cost, description fields are all defined in LineItems and a calculation field can compute and show the cost of that one line item on your invoice.
Fields in Estimates can identify the customer and a calcualtion field can use Sum to compute the total cost of the estimate.
If you only sell custom made tanks, you may find that you do not need the ProductsServices table.