Use the "Inch Rule."
When installing a seat, one of the most important things to consider is how much it moves after install. Hawker says: "The rule of thumb is that it shouldn’t move more than an inch when you push and pull. Kind of tug at it, you don’t have to lean into the whole car seat. If it moves more than an inch, it’s usually not considered a very good install and isn’t tight enough. If it wiggles no more than an inch, it’s considered good to go."