I think you are asking yourself the right question about what tax bracket you will be in when you will retire.
A couple of points to bear in mind.
First have a look at a US tax table e.g https://turbotax.intuit.com/tax-tools/tax-tips/IRS-Tax-Return/2014-Federal-Tax-Rate-Schedules/INF12044.html
The takeaway is that if you reduce your taxable income in any year; the savings to you depends on which part of the schedule this reduction applies. [otherwise known as the marginal tax rate].
So the follow-on point is when you retire, if you are smart, you will match your income to your expenses so that your income is as small as possible and thus your tax burden is as small as possible. You don't need income that you will invest in retirement.
So if your kids are in college, and assuming you own your own house [maybe by downsizing] by then, how much income will you really need? Going further, ask yourself, what will be the marginal tax rate for the tax savings you will realize by not paying taxes at this time [as you would in a traditional IRA].
So if you are making more than that now, and you marginal rate is higher, you should offset this in your calculations to the perceived advantages of a Roth.
So hypothetically, I believe that an appropriate income for myself in retirement is say $70,000. So if I get $10K in tax-free income, the realized savings on taxes is 25%, or $2500; as this is the marginal tax rate.
Not let's say I am currently earning my money in a higher tax bracket; say I make $110K a year. Reducing my tax burden by $10K confers to me a realized tax savings of 28%, or $2800, based on the marginal tax table.
So whether or not to use a Roth or traditional IRA should in some sense be driven by forecasting your income in retirement, as compared with your current income, from a marginal tax rate perspective.