Stuck at making a material

I’m a newbie in UE4 and this is my first post here. I’m trying to make a simple water with water drops on it’s surface. I’ve created some kind of mask for the water drops. Every drop have a value from 0.1 to 1.0. I want to use every water drop, if a global material parameter is above a value (e.g. I have a timeline where I set from 0.0 (which means there is no water drop) to 1.0 (where every drop appears)) and I clamp the texture to that point then ceil it. My problem is that if the value is above 0.1, every drop appears. I want to use that part of texture if it’s equal or smaller than the parameter. I’m sure clamp is not suitable for this. But what can I use instead?

I hope someone can understand my problem and sorry for my english, it’s not my primary language

Please help