It's not that complicated tbh, the way he did it.
Kallis has an average of 56.41 over 244 innings. We'll assign a span of 20 innings, and let A represent the average required over those 20 innings. Now the equation is simple -
[ (244/264) x 56.41 ] + [ A x (20/264) ] = 60
Now solve for A. Although he seems to be incorrect, as according to my calculations Kallis will need to score at an average of just under 104 over the next 20 innings. However, even with an average of 97 over the next 20 innings, he'll be looking at an overall average of around 59.5. Close enough