MRP / Bill Of Materials Vs. Production

I am trying to do parts forecasting.

My customer makes 6 different widgets, all of which use a different number of the same parts (tabA ~ tabZ)
For example, model A has the following:
1 x tabA
2 x tabB
0 x tabC
4 x tabD.

I have a matrix which shows how many of the different parts are in each model. The part number goes down the rows and the model being produced goes across the columns. So this goes from A10:G:21

Now, I have to do a forecast for the next year and I trying to figure out the best way to do it and have each months totals of each part. My forecast has the model numbers going down the rows and the months going across the columns. This is from A1:G7

The way I used to do it is have a line for every part and manually multiply the monthly total by each models per unit and it ends up being a formula that gets longer and longer.

January needs for tabA are (B2*B11)+(B$3*C11)+(B$4*D11) ...
February needs for tabB are (B2*B12)+(B3*C12)+(B4*D12)...

When they have 20 distinct models it gets difficult. Also, there may be mid-season changes to the bill of materials that make it difficult to change.