My son is very close to outgrowing his cute little CCO. What I need to know is, when do I measure him in the seat? When I first put him in and he is sitting nice and straight, he is at the 1" limit. After driving for a bit (we are rarely in the car more than about 20 minutes) and he has wiggled and scooched as toddlers do, he has more room above his head and it doesn't look outgrown. Which measurement matters more?