Is your structure like this:
Estimate Table < Estimate Items
In your Estimate would be your single-entry data such as date, customer name etc. Then in your Estimate Items would be each individual "line item" that forms part of the Estimate. This would be where you would select your category, select a line item etc. assign a quantity and enter or auto-calculate pricing for those line items.
Then you can summarize your total pricing to the Estimate Table where you then get your "total" for your Estimate.
The idea behind the child-table of estimate items is that each item has a single-selection field for each value that creates a unique-line item with its own qty and pricing. I would not recommend building a list of line items in a single table using tens of "selection"/"entry" fields.
I'd be happy to work through this with you offline and give you some guidance.
