The seat you have has that marker set at 36cm from memory. Which isn't very big - it is just the average shoulder height of a 6 month old, 8-9kg baby. Which is exactly what the age/weight limits on the seats were. The conclusion I came to after a back and forth with Britax (SnS) was that the change to height markers was really not much more than a PR stunt, it doesn't mean the average child can be turned later.

We removed the inserts frpom the car seats when they started hindering the position of bubs in the seat.