How Do I Solve This Inventory Issue In Numbers?

I just finished your Numbers course and it was great! Unfortunately, I don’t think what I’m looking for was included(probably wrong on that one). Also, i am still trying to understand the language and logic of the formulas, so that’s why I’m asking for a bit of assistance.
I hope its not too convoluted!
I am trying to make a monthly inventory spreadsheet for my brewery. The front facing table (orders) will have separate pull down columns for client, beer and package (1/2 keg, 1/6 keg, case and 4 pack), with a column at the end for actual amount ordered of that particular beer and package. So an example would be:
Client Beer Package Amount
Brett Pilsner 1/2 keg 2
I have another table with the actual inventory. Our Beers in the header column and 3 columns for for each package (i.e. 1/2 keg Begin (for begin of month), 1/2 Keg Add (for any beer made that month) and 1/2 keg current (which would be the subtracted total of all orders pertaining to that beer and package size):
BEER 1/2 Keg Begin 1/2 Keg Add 1/2 Keg Current
Pilsner 10 10?
That’s where my problem starts. From the Orders table, I would like the add all the order amounts of a particular beer and package size, to be added together then subtracted from the sum of the 1/2 keg begin and 1/2 keg Add.
I realize this is a bit insane and understand if you cant make heads or tails of it. I tried to include screen shots here, but no go.
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Brett Blau