This link is more of a help.
the formula should be:
1 in (187.5 / ( x )) per step
Let x equal the a value which determines how rare the Pokémon is. The higher the encounter rate, the more common the Pokémon is.
There is about a 4.5% probability of there being a Pidgey and about a 4.5% probability of there being a Rattata. This means about a 91% probability of there being no wild Pokemon.