Closingbalancemonth with data from two different sources

Hi all!

I run into a problem using CLOSINGBALANCEMONTH with data coming from two sources (one contains actual data and other one contains forecast). I want to calculate Inventory level and basically I want my pivot to show Inventory for Q1 on the same level as Inventory for March (eg. January Inv.=100, February  Inv.=80, March Inv.=90, Q1 Inv.=90). 

If I sum two CLOSINGBALANCEMONTH functions (one to calculate January Inventory (in this case actual data) and other one to calculate February and March Inventory (in this case forecast)) the result for Q1 is sum of January and March Inventory.

Does anyone have idea how can I solve this issue?

Thanks!