Mob spawning too often; parameters not allowing decimals

Just as the title says!

I have a mob that I want to spawn as a Creature or a Monster would; but I want it to spawn much more rarely than most other mobs. I'm trying to put it down to an 0.2% chance through the software, however it refuses to allow anything below a 1 and simply takes my "0.2" as a "2".

Thanks in advance!