1:20 is half the strength of 1:10, so nothing should have burned, that would be 1 part ammonia to 20 parts water or 1 part ammonia to 10 parts of water. Now, I would think that something like 1:5 or lower could burn the hostas.
Opps, sorry... 25 per cent was mentioned, I am surprised that did not burn them but maybe cooler weather you could get by with that, I would be concerned in hot weather, but maybe not. I try to keep my mixture around 1:10 so I have no experience with a 1:4 mixture.
