I let random.org select a % and it decided on 18%.
“Who are the Darkfells?” Stephen’s mother asked.
“They’re an old, noble magical family.” Madame Veronika shrugged. “Respectable. Their patriarch Eddard Darkfell, managed to obtain the clockwork sundial, which in turn allowed him to amass a vast collection of magic items by traveling through time. He only stopped when a paradox threatened the family.”