Marlowe Quintero spent seven years inside sportsbook and casino promotions before she started writing about them, four of those drafting and checking the terms that sit behind a bonus. That is an unglamorous specialism and it is the one this subject needs, because almost every complaint about a promo code turns out to be a condition nobody printed where the player could read it. She checks whether a code still attaches to anything before publishing it, and says so when it does not.
She writes terms-first rather than headline-first. The percentage on a banner is the least informative number an operator publishes; what decides the value is the rollover behind it, what that rollover applies to, and whether the code attaches to anything at all.