[Help] Fetching Values from Child_Table_1 to Child_Table_2 in Sales Order form

Hello, and thanks in advance!

In our line of business (iron merchants), item prices are calculated by considering three factors:

  1. Base Iron (or say product category) price. Negotiated with each party for each transaction.
  2. Item-making price as per sizes. (Item prices shall be defined in price lists).
  3. Method of tax calculation. i.e., [Base + Item + GST], [Base + GST + Item] or [Base + Item]. Again, this varies for every transaction and may be different for different product categories.

I started by making a new child table (above the items table) in the sales order form where I would populate relevant data to aid calculations. Something like this:

But I’m not able to fetch data from Base Rate Table (Child_Table_1) to Items Table (Child_Table_2). I’ve created custom fields in the Child_Table_2 where I what the data fetched and tried a few things, but no luck.

Please if anyone can share the solution to fetching data and properly calculating the same.