Hmm... You have a 'sums to' relation to satisfy.
It's related to this question, but applied to each row. Have a look at the solutions there, and see which if any you would like to adapt. You're doing that multiple times.
My suggestion would be to let
them enter any figures in any of the
entry fields. You then
flag inconsistencies where the numbers don't
add up. You change the appearance of the fields that are
inconsistent. You show a warning message explaining
People are used to seeing this kind of tabular breakdown with a totals column. You make their life easier if they can enter quantities in that format too.
The pattern here is separating data entry from validation logic. Do that and you allow users through 'bad states' on their way to a valid solution. It makes their life easier and gives you more flexibility in validation. You probably already have other dynamic constraints too, like not allowing any pack to be over 800 Kg in weight. The approach outlined here allows you to have such column constraints as well as the row constraints.
An optional Refinement: If you want to save the user some time typing and some arithmetic, entries they leave blank can be suggested penciled in (in grey) automatically, by splitting the remaining quantity evenly.