As far as I know, it's different for different exchanger types, e.g. 1 shell, 1 pass, 1 shell 2 pass, etc.
But from Coulson & Richardson Volume 6, Thrid edition (p656):
For a 1 shell : 2 tube pass exchanger, the correction factor is given by:
The derivation of this equation is given by Kern (1950).
It then shows the graph, and says that you can find other graphs in TEMA standards and books by Kern (1950) and Ludwig (1973).
Your best bet might be trying to find the book by Kern.
From the reference list:
Kern, D.Q. (1950) Process Heat Transfer
Hope this helps