I aimed for 500K since I remembered in 2012 how Katy scanned 411K with Part of Me after the Grammy's and how Britney had scanned the same amount 1 year prior with HIAM, if history repeats itself, it's possible that Britney and Katy may come close to each other's sales and Britney might scan within the range of 450K - 500K, especially since the digital music market has expanded since her latest 400K+ debut.
But, since I haven't listened to the song yet, and don't know what kind of promotion it will receive on the first week, my safe bet is 300K - 350K.