A simple average of a conference's teams' rating values can be misleading - a conference with one very good team or one really bad team has an average that might not be representative of overall conference strength. Here's a weighted average that gives most of the weight to the middle third of the conference's teams.

For a conference containing n teams, order the teams by ratings ρ(t) to assign each team a rank from 1 to n, let tr be the rth team in the list, then define the conference rating by:
ρ(conf) = n
∑ w(r) × ρ(tr)
r=1

with
w(r) =
( n-1 ) ÷ 2(n-1)
r-1
(x) = C(x,y) =
y
x!
y! × (x-y)!

n
∑ w(r) = 1 since
r=1
n
∑C(n-1,r-1) = 2(n-1) so ρ(conf) is a weighted average with weights defined by conference size n.
r=1

The weights for "conference" sizes 1 through 13 are:
Group
Size 		Teams' Ranks Within Group
1 2 3 4 5 6 7 8 9 10 11 12 13
13 2 29 161 537 1208 1934 2256 1934 1208 537 161 29 2
12 5 54 269 806 1611 2256 2256 1611 806 269 54 5  
11 10 98 439 1172 2051 2461 2051 1172 439 98 10    
10 20 176 703 1641 2461 2461 1641 703 176 20      
9 39 313 1094 2188 2734 2188 1094 313 39        
8 78 547 1641 2734 2734 1641 547 78          
7 156 938 2344 3125 2344 938 156            
6 313 1563 3125 3125 1563 313              
5 625 2500 3750 2500 625                
4 1250 3750 3750 1250                  
3 2500 5000 2500                    
2 5000 5000                      
1 10000                        
(Here I've multiplied the weights by 104 to make the table easier to read.) Graphically,

Weights by teams' ranks