In general, what I did in order to determine the A_V (cluster) was to
use the OB stars listed by Humphreys (1978).  I eliminated all the RSGs
in determining the average, and also excluded all stars for which she
did not list a distance modulus.  I then threw out stars whose individual
distance modulus (i.e., their spectroscopic parallaxes) disagreed with
the cluster averages significantly---these were USUALLY (but not always)
ones that she flagged as foreground or non-members.

Since she used R_V=3.0 in determining the Av's of these early-type
stars, I then divided by 3.0 to get the values listed in Table 2 of
Paper I.  For the purposes of comparing to our Av values for the RSGs,
I then multiplied by 3.1.

Here are two specific examples.  
Cep OB2: 
   discarded all RSGs plus the following based upon DM:
   HD205196
   HD207260
   HD239725
   HD239725
   HD204827
   HD206773

   We are left with 14 OB stars.  The average of their
   Avs is 1.92. The standard deviation for this sample is 
   0.49 mag.
   This Av value was divided by 3.0 to get the
   E(B-V)_Cluster=0.64 value listed in Table 2 in Paper I.
   We multiply this by 3.1 (for consistency) and obtain
   1.98, with a 1 sigma of 0.50 mag.

   For the 3 RSGs in our sample in this cluster:

    star         Av(Paper I)  Delta Av    
   HD202380      2.63         +0.65      
   HD206936      2.01         +0.03       
   BD+56-2793    2.32         +0.34       

  
Cep OB1: Removed:
   HD212312 (foreground)
   HD216927 
   HD217476
   plus all of the stars for which there are no DM given plus RSGs.

   We then find an average A_V=1.76 (R_V=3.0) based upon 27 stars; the
   standard deviation is again 0.47, 
   --->E(B-V)=0.59---A_V(R_V=3.1) of 1.83 with sigma=0.48 mag.  There are 
   two RSGs:
   Case 75:    6.05          4.18       
   Case 78:    4.65          2.79      

   So, I think it is clear that these stars are significantly 
   reddened (8 sigma and 5 sigma!)

Here are the details for the complete sample of clusters.
  
          Ave   sigma    N   
Cas OB4   2.25  0.66    19   H78
Cas OB7   2.60  0.57    11   H78
NGC457    1.59  0.13     3   H78
NGC581       See Becker & Fenkart 71
NGC663       See Becker & Fenart 71
Cas OB8   2.17  0.61     7   H78
PerOB1    2.07  0.62    57   H78 
N884      2.03  0.30     3   H78
Cam OB1   2.16  0.47    13   H78
Aug OB1   1.64  0.45     5   H78
Gem OB1   2.27  0.67    13   H78
Coll 121  0.06  0.05     3   H78
N2439     1.72  0.49    13   H78
N3293     0.93  0.09     2   H78
CarOB1(al)1.68  0.75    25   H78 0.81 Av in Paper I a mistake