Take in mind the event gacha pool is only refered to SR cards, not R ones. Grabbing R slayers and other R cards doesn't clean the pool and chance bonus is to take a SR card regardless if is a slayer card or not.

Based in chance bonus scaling, you will waste 5 egacha to see a SR in the worst of cases. This means you will clean only 4 SR cards in the worst of scenarios, but maybe you will see R slayers in the way.