Depending on how much you earn and how much already have in your 401k, your $2.7 million number may be in error.

Taking your assumptions at face value, I just ran a quick calculation, seeing how much you need to earn per year to go from $0 at age 28 to $2.7 million by age 55, assuming you invest 5% of your annual income and earn 8%/year.

The answer is $524,400/year income.

If you're earning $524,400/year, and can only set aside 5%, then you're probably spending way too much to be able to retire on $2.7 million, or even double that.

More likely, you're earning nowhere near $524,400, and somewhere in your calculation that shows you getting to $2.7 million in 27 years with 5% of income invested at 8% return, there's a math error.

Of course, you may already have a healthy 401k balance. To see how that affects things, if you have $100k in your 401k at age 28, then kudos. However, that reduces the required annual income only a little bit, to about $516,000, to make your numbers work out.