I usually ask for the canter at the beginning of a corner but when he picks it up himself it is wherever he wants to pick it up. Am I picking it up where I am supposed to? Or should I pick it up at a different spot?
As I suspected.
So you ask for a canter when there is a nice huge space for the horse to have.
This is what separates the beginner riders from those that think ahead.
A straight line is your horse's friend...a curved line is YOUR friend.
So asking for a canter when coming into the short side ( or ends) of the arena limits the space available for the horse to run...count advantage to the rider. This also give the rider time to make adjustments should they choose to proceed down the longer side where the advantage goes to the horse.
Be prepared to circle ANYWHERE along the long side to bring back the advantage to you. If the horse breaks canter DON'T rush it back but PICK your spot where YOU have the most advantage. And use the trot to gain control again.
This is called thinking riding.