Rolling average across columns with criteria

[Lee1800]

I am urgently needing assistance have been trying to use previous assistance but does not appear to work for this.

I have the following data and would like to be able to avearge across the row for each employee. I would like the managers to be able to selet from validation list the average period ie. 3,4,5.

I have attached a sample of the existing report

Any assistance is greatly appreciated

Lee