If my ETS balance is 42.5 days, and I get out in February 2018, am I allotted all 42 days for transition/terminal leave?

Well you have your answers, bottom line up to Commander, I myself put in for over 60 days when I retired. I had 60 on the books and earned leave while on leave, It was approved.

Leave time is yours and your alone.
The commander will have to approve anything over 60 days, by regulation if you are not flagged you can take leave,
terminal leave is the same way , it’s your call on the way you take it, use it, or sell it back.

You will also earn another 5 or so days between now and then, along with what you earn while on leave.
Most units limit leave to 30 days taken at a time. In order to avoid losing any or selling it back, you need to talk to the folks at the transition office and your CoC.

30 days was always the rule that I experienced. I always saw people take 2 weeks, and then come back for a couple days to clear, and then take their terminal leave. A lot of it depends on your chain of command. If you were retiring they’d be a lot more lenient. My advice? Take the 30 if that’s all they’ll let you take and sell back the balance.

Why are you asking for an 18D or 18F to advise you about Admin crap?
If you have unused leave you may cash it in upon ETS./ S//

As another service member stated: command descretion. Unless you’re getting a stop loss (doubtful), you either get out on time and take the leave as your salary, or you take however much leave you have before your ETS or EAS. So 42.5 days before you are scheduled for discharge would be when you go on terminal (leave to don’t return from).

You are allotted what your command allows. I took a full 60 days when I got out. Not sure if it's the same for the Army, but be aware that I had to route a Terminal package a full 30 days before I was due to take Terminal, so you might be coming up on a deadline soon which could delay your Terminal start date.

depending on unit SOP over 30 days requires either your BN or BDE commander.

Yuppers. As long as your command approves it and you have everything already cleared.

Generally, yes you should be able to take all 42 days. Sometimes an amount that high requires sign off from your chain of command, perhaps a senior officer. But in my limited experiences with this you can get it through as long as your personal CoC is supportive.