I am installing (myself) two 6500 cfm MasterCool Evaporative Coolers
into my 2800 square foot house so I don't have the expense of using
the existing refrigerated air system.  I know the refrigerated air
ductwork is undersized for the volume that evaporative cooling uses,
so I will be installing a completely separate ductwork system.  Using
rules of thumb, the combined 13,000 cfm of the new system should be
overkill even in my part of the country (Mojave Desert).  The house
will be zoned into two halves, each with its own thermostat and
MasterCool unit.  It has a plain flat ceiling 8 feet high throughout
(no vaulted areas)  My question is:  Knowing I have intentionally
oversized the system, is there a quick and dirty rule of thumb for how
the ductwork is sized throughout the house?  In other words, I expect
the duct diameter to shrink as rooms are fed to maintain velocity and
pressure.  How should I figure this shrinkage?  Or is my supply so
great I can just run 10 inches to each duct and forget about it?

My own net research has led me to programs like DuctPuter, LoadCalc,
and HVAC-Calc, which appear to be more geared towards calculating the
total house heat loads and sizing of HVAC equipment to meet those
loads.  I don't believe I need that complex of a solution:  My house
is already built so I can't change any of the variables (room
geometry, window configuration, R-Values, etc) and I know I have
oversized the CFM of my system so an exact calculation of my heat load
and the equipment to service it is not required.  I just need to know
how to reduce the duct diameter as the various rooms are fed.

Some additional info if it's helpful:  Each MasterCool unit will enter
a plenum and transition to an original duct diameter of 20 inchs. 
Each unit will service five outlets and the total run from each unit
to its final outlet will be no more than 40 feet.

Bonus hearsay question: I heard a rumor that collapsable insulated
ducting
doesn't hold up to the higher pressures of an evaporative system over
the long term - anybody else heard that? (this is not an important
question).

Thanks,

Dan