Unfortunately in the current Sage 200 Professional API there doesn't appear to be an endpoint that would provide an approximation of the data stored in dbo.StockItemPeriodBalance. Altough these tables have great data within them.
One example,of Period stock values are that currently: Period stock values could be calculated from data on the Product_transaction_views endpoint, however, involves heavy computational resource. This data is already available within tables and offering this within the API would drastically impact performance and allow developers to deliver greater value.
It would be greatly appreciated if such tables could be included in the end points, or perhaps certain KPI data pre-calculated from these tables and I am sure many other developers would be thankful for this. Hopefully at some point this could be identified and perhaps become a prioritised item for future releases.