No, the formula is the same.
>1 in (187.5 / ( x )) per step
Now, the encounter rate's formula is the same, but Pokemon is caught depending on how rare it is. Bulbepedia has a chart that determines that.
Also, the different types of grass detemine the encounter rate. (ex. Audino is very common in shakin grass, but impossible to find in regular. Not, sure if that's true, just an example)
Hope This Helped :D