Unfortunately my HSA custodian isn't willing to calculate this. They said it's up to me to figure out and request the correct amount. I've asked my employer to initiate the distribution so the payroll taxes get withheld properly and it shows in my W2, as you stated. The NIA calculation was the piece I was missing.
It my payroll department and/or HSA custodian were more helpful, this is exactly how I thought it should get resolved!
