I got a world that dates back from 1.0 or 1.1 that's 52% corrupt. I didn't even try.Entirely corrupting the whole world is indeed not enough; you need to either add tremendous amounts of blocks to your world that are corruptible, or remove a huge amont of non-corruptible blocks from your world.
In my world, I tore massive swaths through the ash and solid mud of the jungle, and added single strips of dirt which I grew corrupt grass on. It took many tens of thousands of removed blocks to get to 51%.
I'm on a small world, I Hallowed everything I could, it got me to 40%Ash and Jungle are your two easiest targets; enough destruction of them should be more than enough. You DO need to Hallow the rest of the world, that is important.
I hope you aren't doing this on a large world? That could complicate things.