The $3600 is your taxable income, so you'll owe 10% of that, or $360, in total tax. Deductions (like the standard deduction) are subtracted from your income before you calculate your tax. Credits (like the savers tax credit) are subtracted from the amount of tax you owe. Since you'll qualify for the full $1000 savers tax credit you actually subtract that from the $360, but since the Saver's tax credit is non-refundable it can't make your tax due negative, so you just end up owing $0. This is why I'm suggesting you switch some to Roth. You want to "use up" as much of the savers tax credit as you can without making yourself ineligible because otherwise you're deferring income (paying taxes on it when you withdraw later) that you would have paid no taxes on otherwise.
The ideal situation is to have an AGI (line 38, form 1040) of $19k because then you're still eligible for the full savers credit, so your taxable income would be $19k - $12k = $7k, so your tax due would be 10% of that, or $700 which would also be reduced to $0 by the $1000 savers credit. As you can see you can't actually maximize the savers credit because getting under the AGI limit makes your taxable income after the standard deduction too low, but still it's a pretty good deal.
Not to complicate things, but you could also look into the
Earned Income Tax Credit. I don't qualify (need pretty low income when you don't have kids), but you might be able to. To qualify as a single filer without dependents you need to have both and AGI and an earned income below $15010 (in 2017, I don't know when they'll release the limits for 2018). The credit changes based on your earned income amount. I messed around a bit with the calculator found on in one of the links on that page I linked to and it looks like you don't get anything if you get JUST under the $15010 limit, but if you get down to $10.5k ($29k - $18.5k 401k contribution) you'd get $343. Unlike the savers credit, the EITC is refundable, so since you'd be below the standard deduction ($0 tax due) you'd actually get that money back.
Since you'll owe $0 either way, what I would do is consider that $343 the "tax" you would pay on the money you would put in Roth if you didn't try to get the EITC. Since you're comparing a $18.5k traditional TSP contribution to get as much EITC as you can to a 10k traditional TSP contribution to get the full savers credit that's $8.5k. $343/$8500 = 4.03%, which is a pretty low tax rate, so it seems to me that not trying for the EITC, and just putting enough in your TSP to get the full Savers Credit with the rest going to Roth is the way to go. It might be worth playing around with the EITC calculator yourself though. There were some things about combat pay that I didn't really understand since I don't know anything about how that works.
What I would do at this point is switch your TSP to Roth and just leave your IRA alone. Once the year is done you can do your taxes with your actual final numbers and see how things shake out. You can recharacterize IRA contributions between traditional and Roth (in either direction) any time up to the tax filing deadline for the tax year towards which your contributions were applied (so 2018 IRA contributions can be recharacterized any time until mid April 2019). This way you can fine tune your AGI to be very close to the $19k savers credit limit without going over.
Make sense?