The center is flat and the beginning of the tread is pretty much flush with tire. Was hoping to make a 6 hour trip but not sure how much rubber I have in the center. The sides still have plenty left. It's crazy my tire only lasted 4 months.
Tires generally have a wear indicator, which is to say a small piece in one of the tread channels, where if the piece inside becomes flush with the outer edge of the tire (thus the tread line is completely "broken"), it's time to change the tire.
Don't mess with a worn tire. I had a blowout when riding my 1987 CH150 Elite scooter on the freeway at full throttle, 65 mph, in front of a pickup truck. The bike starts to sway side to side more and more until the point where it's going to flip over. I was able to shimmy it down to a decent speed then turn it onto an embankment right at the point it was going to flip me over. The guy in the truck stopped and said, "I need to shake your hand. I don't know how you did that! I thought for sure you were going to be thrown off and I'd run you over."
Remember when you get a new rear tire that A) the bike will suddenly turn so much quicker than you've been used to -- with changing a rear only it will feel a bit like the back tire is sliding out from under you, and B) a new tire is very slippery to start with, so you could easily start sliding it for real.