The step size should satisfy the condition:

0 < step-size < 1/(max eigen value of the input autocorrection matrix)

For fast convergence step-size is set close to its maximum allowed value.

If you choose very big step-size, convergence is fast, but final error is also large.

I would suggest, first use maximum allowed value and once abs(change in error) is stabilized, keep reducing step-size to reach the best result.

You can put 'if' loop indicating this condition in your program.

You may find the discussion in the following chapter useful:

http://www.ece.utah.edu/~mathews/ece6550/chapter5.pdf