she is stealing from the company, her colleagues, and the shareholders. Report it via anon letter.

You say you figured out the issue. Are you positive you have the right formula?

I would approach your manager with it, of its not resolved, I'd send a letter to the CEO and board.

In short, I'd move to get this person fired immediately.

I think you have to report - soooner rather than later. She has compromised the intergrity of the compensation process and you have to give your company an opportunity to fix it. You have to "protect the fairness of competition." If you don't, you are only slightly better than she it.

Also, if you do NOT report it or report it later, do not let on that you discovered it in April or that you otherwise knew of the issue. If you do, I suspect that you will be fired.