Numbers for TSP and IRA are still $18k and $5.5k, respectively.
Also,
I found this. Nothing I've seen in TSP will stop you from accidentally going over. It was easier when you could put a dollar amount instead of a percentage contribution, but they changed that recently. Of course, it comes out of your base pay only, so you could theoretically sit down with a calculator and figure out how much to contribute as a percentage of your Jan-Dec paychecks ($1500/monthly base pay). Of course, you'd have to update that percentage yearly so you don't over-contribute. Which is dumb.
I would just figure out a percentage of your paycheck that's a little less than 1500/month, maybe round down to a nice whole number, and just go with that. For example, "my friend" who's a JG passes the three-year mark around June-July, so his base pay will be 3900 for roughly half the year, 4492 for the rest of the year. That's about 4200 on average. 1500/4200=35.7ish%
You'd still have to keep an eye on it every year, but that's what one of the sections on the LES is for: seeing how much you put in TSP this year. So just check on it in like October, and adjust as necessary. If you go over, no big whoop, see article.
IRA is relatively easy, just set up an allotment from your paycheck, or a scheduled transfer from your direct deposit checking account, for $458.33. You'll be about 4 cents short of the contribution limit.