I got the NADA retail value for my car and did a little research online for similar cars. When the insurance company called me with their offer, I told them that retail on my car was 12k and that wasn't including the fact my car was meticulously maintained and I had done a bunch of preventative maintenance on the car that they hadn't taken into account since most cars on dealer lots would not be as well maintained. They told me that maintenance isn't figured into the settlement because they assume all cars on the road are properly maintained.
I then hit them with a bunch of cars.com listings for cars that were priced around what I was expecting. They later told me they could only offer me 10,800 for my car + taxes but they're write me a check for $500 for "personal loss" because i was adamant that my car was exceptionally rare and hard to find.
My tip is to be firm with the adjuster, but don't be a jerk. They're just trying to do their jobs. Make sure you have a good amount of evidence to back up why you should get the extra money.
And of course, make sure they're giving you a suitable rental car for the time being.