Question: 

When a tank or compartment has a deducting component, what permeability
should I use to get the a given net permeability of, say, 0.85?

Answer: 
    
In order to arrive at an overall permeability of 0.85 the deductions would
normally be 0.85 also.

The default permeability for additional components (adding or deducting)
is the same as the permeability of the first component.  This is true
whether a component is produced directly or by reference to the component
of another part.

The paragraph in the manual which reads "If the component is a deducting
one, it still is assigned a permeability; however in this case ..." is
only an apology for using the term "permeability" for a deduction when in
fact it would be more logically called "impermeability".  It was not
intended to imply anything about the use of any particular permeability
values.

There are two ways that permeability can be used:

1) The formal, regulatory, or customary permeability values are used
without further consideration of the actual structure.  This is what
we have been considering so far.

2) Calculation of the volume occupied by structure, machinery, fixed cargo,
etc. leads to an average permeability for the space.  In this case, the
permeability used for a deduction will depend on the nature of the object
being deducted.  If it is a smooth tank which does not overlap any of the
structure already deducted from the overall space, then its permeability
should be 1.0.  It is in effect reducing the molded volume but not the
structure volume.  Therefore, the average permeability must decrease as
a result of the deduction.

The PERM command in the GHS Main Program is a convenient way to check
the average permeability of a tank.  It can also be used to set the
permeability of a tank (assigning the same permeability to all components).
Thus,

  PERM (ENGRM.C) = 0.85 

insures that ENGRM.C has the permeability of 0.85.