How do you want the percentage to be calculated?
Since its a matrix report that means you have a dynamic Column and Row so first you need to arrive on which basis you want to achieve the percentage and then apply the formulas accordingly. if you just want to have % field overall then you can
just do something like = value/(CountRows("DataSetName") *100.
So the answer would be depending on your requirement.
If my response answered your question, please "mark the response as an answer" and also "vote as helpful". Regards, Hari. www.crm2011byhari.blogspot.com