Hello, I was curious what was the best practice for Mavic batteries. I have 2 batteries which are new, however I cannot seem to get over 18 minutes of flight out of them. Should I fly the battery under 10% then charge, or never go under 30%?

You should leave enough juice in your battery for a safe return in the first place.

...Then think about the battery itself.
Li-Poly batteries like it best when neither super fully charged nor fully discharged. They can live forever, if you just use them let's say in between 20-80% of their capacity.
Now we don't know if the displayed values refer to full capacity or are somewhat limited already (like in electric cars)

In what conditions are you flying and how are you flying? Lots of sport mode and / or hovering in high winds, for example?

Flight time depends very much on how you fly and in what conditions. In what I'd call a "normal" flight -- light to moderate winds, no sport mode, etc. -- I consistently get 20 or more minutes of flight time out of all my batteries, and 22 minutes seems pretty typical. I've gotten up to 24 minutes and still landed with a reasonable amount of reserve, so DJI's specs seem pretty accurate to me.

I've read a couple of times that with brand new LiPo batteries it's best to only drain them down to 50% for the first handful (10 is sometimes thrown out) of uses. I don't know if there's any truth to that, but there doesn't seem to be any harm in it, so I've tended to follow that rule of thumb.

I'm assuming that you're referring to the 10% threshold that's supposed to "reset" the battery. You definitely don't need to / shouldn't try to do that on a routine basis, especially since at 10% (or higher, depending on your settings) you'll get a critically low battery warning, at which point the aircraft will insist on landing. If your batteries are new then you shouldn't need to do this to get the expected performance out of them; at least I never did.

I've read that for the first 10 uses you should run them down to around 40%. After that don't run them down to less than 20%. Always store them at around 30% or 2 lights