I have no idea where these 15% and 25% numbers you speak of are coming from. As far as I know the bonus rate used is completely uncharacterized--that is, we have no info at all on it.
If we had 15 or 25% chance of pulling an event SSR for each Rate up, we would have each hime about now.
Looking for a kinda good account? Send me a Private Message or meet me at Discord: L'aventale.-#4530