For this, this is up to you:"Specifically, are you using Inventory Items or Non-Inventory for your WIP materials?"
If you manage it as inventory, individually by Quantity, you also know there is Cost Basis tracking per unit, which sill be average or FIFO, depending on your program and settings. If there is one Overall cost, such as I have a huge import duty and broker fee, I might use an Other Charge type item, or Two, to track Broker Fees and Import Duties as those individual WIP items, to show a total incurred for each type of cost is going to need to be fully invested in the end production run for this Lot and allocated as part of cost per unit completed. Example: The order is for 15,000 golf balls, and when all the shipping, inspection, prep and packaging is done, the total cost is allocated into 14,855 balls. The rest were Scrap.
"When you do the Inventory Adjustment to add the finished product to inventory, does there also need to be a second adjustment to adjust the WIP items?"
Only if they are Inventory. Let's review that WIP = an Asset. Inventory = an Asset. If you track WIP as something to be counted and managed, then yes, the Adjust Inventory Screen would allow you to offset them by item name. Example:
QB Pro does not have Builds. So if I stock various table legs as I make them, I would use a Raw Materials WIP account to hold the purchase of raw stock. When I produce Legs, I use Adjust inventory to put the legs as finished products on hand with qty, of the various type I produced, and top left, I clear the Value from WIP as a generic Total. Later, I assemble some Tables, and I can use Adjust inventory to Reduce the count of the specific Legs and Tops, if I made the tops in advance. Or, I populate Tables with Qty, reduce Legs, and pull the remaining value for this production run from the WIP account where I posted the purchase of Tree Slabs that I used to make the Table Tops in this production cycle. Now I have fewer legs, less raw materials, and a Count of completed tables, fully costed and ready to sell.
I teach this as: If you can map out what happened, we can use QB to show it happened that same way.
"For the adding entry, we would use the COGS as the Adjustment Account"
Not that I see in this discussion. Cost of Goods Sold in populated from the Sale. Not from anything in production, other than, you might need to throw out waste or account for shrinkage.
"and for the subtracting entry we would use the WIP Other Current Asset as the Adjustment Account, yes?"
No. If you put something Into WIP, you eventually should show it was used and is still on hand as this Other Thing, or sold.
"On the other hand, when you say you create a $0 Bill, do you mean that you are balancing the costs between the Expense tab and the Item Tab?"
Don't think of this as "cost." It's Values. It's a data entry worksheet for offsetting values. Example:
That same Table production run...
WIP Bill
Negative 4 legs on the Items tab
Positive 1 Table on the Items tab
Pull any WIP Value for raw materials for the table, from WIP account, on the Expenses tab as Negative
hit Recalc
Should = $0
Table is now Fully Costed, WIP is reduced by Table top Raw Materials used, 4 legs are invested from inventory to Table basis.