The Index for pricing in the PERC contracts is based on the average of the 
"spot" price and the "citygate" price as follows:

Calculation of spot price (C):

C = (A+B)/2
Where A is the low and B is the high of the range as reported in the first 
publication of the billing month of "Inside F.E.R.C's Gas Market Report" for 
Chicago citygate.

Calculation of citygate price (E):

E = (D+C)/2
Where D is the price as reported in the first publication of the billing 
month of "Natural Gas Week" for Delivered to Utility, Midwest.

Calculation of Contract Price (F):

F = E x applicable percentage