Subject: Re: Covered call?
By selling the put you have chosen to pay no less than 449.50 no matter how deep the dive. And you will pay more if you buy back the put prior to expiration. I assume that's what you mean by "exercise", because as the put seller you don't exercise anything.

Of course, I misspoke! You can choose THE PRICE at which you are willing to buy stock (by choosing the put that results in that price) sometime in the future if the holder of the put exercises it, and you get assigned!