Lookup Visual Calculation not working correctly

Gurus - I am in need of assistance.  

Hopefully the table above is showing up.  I'm working on a matrix visual, each month going across with various details in each row.  I need to create a field, ActualInvCalc, for each month that is based on this calculation, prior month's actual inv plus current month's actual sales minus current month's actual prod. 

Using the example data above for 7/1, it would be 6/1's Actual Inv plus 7/1's Actual Prod minus 7/1's Actual Sales. 

ex. 1243000 + 65835000 - 2436500 = 64641500. 

Then for the following month, 8/1, I need to start with what was calculated for 7/1 plus 8/1's Actual Prod minus 8/1's Actual Sales. 

I'm using this calculation - ActualInvCalc = LOOKUP([Actual Inv], [MTHYR], PREVIOUS([MTHYR])) + [Actual Prod] - [Actual Sales]

It's working for the 1st month, 7/1, but it's not for the other months.  For 8/1, it's still going back to start with 6/1's Actual Inv instead of what was the result for 7/1.

This can be done in excel, but my users want to see this in PBI.  I've never used LOOKUP before, so any suggestions would be greatly appreciated.

TIA!!!!