How to report amounts of partially received purchases?

What am I doing wrong here? And more importantly, how can I do this correctly?

Ok, here's an example scenario:

Let's say it's the middle of a calendar month. I want to use Navision to provide an estimate of the amount of purchases for that month.

1. I can use report Vendor/Item Purchases (report ID 313) with filter Posting Date to find out the amount of already received purchases during the first part of the month. (Or should I use Document Date or Valuation Date instead?)

2. I can use report Inventory Purchase Orders (report ID 709) with filter Expected Receipt Date to find out how much I can expect to still arrive during the latter part of the month.

One big problem I had with this approach was that there are partially received purchases: their amount on back order appear correctly on the Inventory Purchase Orders. But, the already received part (i.e. non-outstanding part of the purchase order) does not show up on the Vendor/Item Purchases. I am guessing that this happens because the report only takes into account purchases with zero outstanding quantity?

Naturally I want to also include the amounts of the purchases which have been partially invoiced but still have an amount also on back order.

What kind of approach should I take to get where I want? I did search mibuso for an answer and also spent plenty of time going through different reports to try to find a solution, but wasn't able to.