Formula with lots of variables

In the attachament I need to calculate de the column "I". The formula mus be like this in the cell I3 (c3/(sum(k3:k4))*k3 and in cell I4 must be like this : (c3/(sum(k3:k4))*k4.

But they are not allways 2 rows to calculate, sometimes are 3, sometimes is one.
Variables are : shift, product, and Pcs

Can somebody help me?