What you have done is also fine and will give you the desired results but it would be better to use Journal Entry instead of Purchase Invoices for starting balances.
The method you followed
Skip point 1
2) create journal entry and check items and qty columns. Enter Qty for each item. (If enter value of each item as debit that would be better if not thats also fine because in step 3 you will tell programme the avg cost per item)
3) what u did is correct.
These 3 steps will give you inventory on hand value on balance sheet.
Also