At 0.5 the reading is still ok. If it stays under 1, you only need to do a small water change today.
If the reading is at 1 it's good to do atleast a 50% water change to bring it down to 0.5, that way there is room for it to go up again.
If the reading is at 2, a 50% water change would bring it down to 1, but since you have to consider that it will rise again before the next water change, you need to change more than that. 75% would bring it down to about 0.5.
If the reading is at 3, a 65% water change would bring it down to about 1, and a 85% water change down to about 0.5.
If the reading is at 4, a 75% water change would bring it down to about 1, and a 90% water change down to about 0.5.
If the reading for some reason gets up to 3 or 4 it's probably easier to do two big water changes than try to get enough water out at once. And then do the tests again to make sure the ammonia is low enough.
If you test the ammonia again later today, you can get an idea how much it goes up in a day since you know it was 0.25 yesterday (try to test it at around the same time as yesterday). So for example if it would be at 0.75 later today, you know it goes up about 0.5 in 24 hours. Then you know you need to get the ammonia level down to 0.5 or under in order for it to stay under 1 at all times.