If you're looking for a starting point, take a look at purchasing the Ingenix Audit Tool.
Keep in mind that just "base numbers" rarely seem sufficient. I employ the theory of valid random sampling. You need a target. Some folks are happy with XX amount of charts per provider per quarter, or whatever. Statistical validation adds a whole chunk of weight to your findings.
I would even go as far as to say that if internal auditing was part of your compliance plan, it should certainly be spelled out to have basis in statistical validation. In the event of any future interventions from payer sources or other outside audits, your findings can lay in direct dispute of that, with statistical validation. Of note, payers are employing statistical validation when they're analyzing your claims, why not make use of the same tool?
Check out that Ingenix product. I think it might educate you and help you to create something useful in terms of audit.
Good luck.