I am trying to create a simple database help me make a model railroad freight
car roster that is representative of an actual real railroad.

One table, "Kits", is a list of available model freight car kits.  Another
table, "Prototype", is a list of the freight cars the real railroad owned by
railroad numbering series.  One column shows the quantity in each series and
another shows the appropriate kit from the "Kits" table.  Multiple series can
be modeled with the same kit.

I have already created a query that shows the kits by group and shows the
sum of the quantities for each kit.

Here is where I get stuck.  The real railroad has tens of thousands of real
cars and I do not have the time or money to model them all.  Say I want to
have a model roster of 1000 cars.  I want to add a column to my query that
multiples the sum of quantity for each kit by 1000 and divides by the grand
total of all the quantities.  How can I get this grand total of quantities
from my "Prototype" table into my calculation?