If Turbo Tax notified me that my former employer withheld too much Medicare tax wages and tips, how do I determine what my employer should refund me?

What are the exact amounts in boxes 5 and 6 on your W-2? Please double-check that the amounts you entered in TurboTax are exactly the same as the amounts on your W-2 form. If box 5 is not more than $200,000, box 6 should be 1.45% of box 5.

Form 843 is not a request to your employer for a refund of the excess tax. It's a request to the IRS to refund the excess if your employer refuses to refund it. There is no specific form for requesting a refund from your employer. You can contact the employer by any method you choose, but it would be best to do it in writing, rather than a phone call, so that you have a record of your efforts to obtain the refund.