Re: how to find viscosity of a mix of pastry, to predict it?

By pastry, I assume you mean the dough before it is baked?  If you mean the
pastry after it is cooked, then it is really more like a solid than a
liquid, and so the concept of viscosity no longer applies.  Instead you
will want to characterize the tensile strength, modulus of elasticity, etc.

Viscosity measures how adjacent layers of flowing liquid resist relative
longitudinal motion.  It is a measurement of the shear friction, in
essence.  Dough can be considered a liquid since it will flow in layers. 
Granted, it is a very viscous liquid, and its viscosity varies sharply as a
function of how wet it is, what other ingredients are present, how much it
is stirred, etc.  It is important to recognize a difference, however.  In
normal liquids like gasoline, water, etc, the viscosity is created by what
are essentially Van der Walls (VDW) forces.  That is, the molecules resist
motion primarily because they experience weak bonds forming and breaking as
they flow past.  In dough, however, there are VDW forces as well as long
polymers entangling with one another.  This completely alters the
theoretical framework for calculating viscosity since it involves knot
theory, and is incredibly complex (since doughs have many types of polymers!)

So how is it done these days?  Empirically.  The food industry is actually
very interested in the viscosity of various doughs because they require
this knowledge to design robotic food processing plants.  Consider the
automatic generation of bread loaves.  The dough may be automatically
squirted through a nozzle into a pan, and then robotically moved into an
oven for an exact period of time, etc.  Therefore, there are a number of
statistics that the industry pays close attention to including the
viscosity.  They measure viscosity using a device called a farinograph
which essentially stirs a rod through the dough and measures how much force
is required to stir the rod.  If you wanted, you could build one using an
old fashion hand-held drill, and a spring to measure the moment you are
applying to the handle of the drill.  Or you could buy one from Brabender
(http://www.brabender.com).  The viscosity from a Brabender farinograph is
measured in BU: Brabender Units!

The following links may be helpful:

An explanation of some of the stats involved in measuring dough, as well as
the viscosity using a farinograph:
 http://www.cooknaturally.com/DetailLotInfo.html

A detailed explanation of mixture viscosity prediction including Grunberg’s
equations.  It is applied to alcohols – which do not have polymer tangling:

Canosa J, Rodriguez A, Tojo J.,  Predition of Dynamic Viscosity of Mixtures
With Alcohols,  Chemical Engineering Department, University of Vigo, 36200
Vigo, Spain.  E-mail: jtojo@uvigo.es http://www.deq.uem.br/biblioteca/deq/Anais/CobeqXIII/pdf/877.pdf

Md Zaidul I. Abd Karim A., Manan D., Ariffin A., Nik Norulaini N., Mohd
Omar A.  A farinograph study on the viscoelastic properties of sago/wheat
flour dough systems.  Journal of the Science of Food and Agriculture,
Volume 84, Number 7, May 2004, pp. 616-622(7), Publisher: John Wiley &
Sons, Ltd.

The Faironograph Handbook 3rd edition (a book on the subject!) http://www.aaccnet.org/bookstoretitles/50376.asp

Hope this helps!

Zack