Inventory Adjustment as Opening Balance

As we manually fix stock status of a B/P, what is the best way to upload the quantity on stock as of 31/12 last year.

For sure, we will be using IA for this. But how we can get the quantity on hand from the system for a previous date.

Also, for example, if the qty on hand as of 31/12 is -500 ( negative, as no IN recorded through the system) and the qty we need to fix is 200, can we first make the qty to 0 and then 200 or can we make a total of 700 to make -500 to 200. Which one is better ?