[2021] Poison effectiveness rounding

Hello, and sorry if I missed this somewhere obvious.

Do I need 50%+ poison effectiveness for poison to reduce boss res by 3% up from 2%, or will the poison apply fractional res reduction, like say 2.4% at 20% effectiveness?

Saw a certain pair of unique gloves that gave me an idea, and I'd like to know whether I need perfect rolls for the idea to function at all.

Thanks!