What is the best way to write indents for BEM blocks?

Question

HTML

In general, I was advised to write indents for bem blocks like this: 1) Is it a good practice to do this? 2) If the indents change in the adaptive, is it normal to write this?:

<div class="bem-block mt-20 mb-40"></div>

.mb-40 {
    margin-bottom: 40px;
    
    @media (max-width: 767px) {
      margin-bottom: 20px;
    }
  }

After all, not all blocks with a class mb-40in the adaptive can change the indent to a smaller one, and there will be confusion ((