I just went through this with my daughter. I filled out the ACA with 4 family size, but went in to do a life event when DD got a job after getting a professional job. So I now am at family size of 3 (only have 3 exclusions on the 1040). So she will claim herself, and her income is not counted on my ACA calculation. There is some complications about how to do the split calculations. I have studied it, but there is an IRS publication that is not available yet. I may be presuming too much, but I would think it would be logical (x months with 4 get pulled out).
Concerning the IRA, you can reduce her income, but I presume that she will be working a "short year", so I would pay taxes (15%)? and chunk savings into a Roth. But the point concerning her income, if you don't claim her, and she gets new insurance from her new employer, her income doesn't need to be in your " family" from an ACA standpoint.
