Question about defining the unit

Hi all,
Currently, I am doing a project which is about book production. As all know that, book produciton involved a complicated unit changing logic.
Simple flow diagram of book production is,
arrival of printed sheets------>different part of printed sheet gathered----->combining with book cover
1. I notice that there is always a unit change inside,
Take a book which is composed of 5 parts as an example

a number of sources arrived at the site ( 20 pallets )
Arrival of printed sheets ( 5000 sheets on each pallet)
different part gathered ( there are totally 5 parts in order to produce a book)...
here the unit had been changed already

2. There are a lot of queues in between the upstream and downstream production line, as the size, number of pages of book varied from order to order, the capacity of each pallet hence is not a constant.

3. There are a lot of movable storage unit (in fact car) to store every upstream items, again, the same question, the capacity of the car "changed" order to order. Because the pages of books changed due to different required from customers.

In fact, I'd already construct this model already, what I am frustrating is the alogrithm of this model( average value? ).
I figured out some feasible solutions
1. Making assumption based on real data( standized all the value )
2. Making use of actual value
3. Taking average of the collectied data

And finally, book production involved a lot of robotic facilities already, I am always wondering is there any variable I can discover in the production line. Becasue the processing time is already fixed. Here, if the source(input) is also average, that means, it seems that there's no other variable to cope with. Please hlep me to brainstorm this question, I tract in this obstacle for a while already. Thanks all!

Jason