Ken is right, the 25r is a 20A battery. Although it will work. If you don't build below .21, you will always be below 20 amp and continue to drop, as the battery loses voltage as it drains. And your mod + atomizer is going to have a bit of voltage drop too, so you won't really even be hitting 20 amps on a fully charged battery.
So, yes, a higher resistance would be even more foolproof, especially if you are not experienced in coil building. But the 25R is a great battery and will fire a .21 ohm build without issue.