Grayscale images as sculpt brush height maps

So I’m trying to sculpt some texture into my models to bake out as a normal map. When I sculpt a bit here and there the faces of the model inevitably get pushed around and disrupt the model too much. I’m hoping there is a way to use a grayscale image as a heightmap so that the pixels with values under or over a certain threshold will be pushed in different directions? e.g any pixels with a value under .5 .5 .5 will be pushed into the mesh and pixels with a higher value will come out of the mesh. I’m sure this feature must already be in blender somewhere, could somebody point me to it please? Thanks a bunch chaps.

what is the basic shape of your model?

can you not just apply some texture as a normal map or a displacement map?

I could do that, but it wouldn’t be the same quality, baking the normals produces more accurate results and makes it easier to texture. Besides, I think this is a pretty common feature, I just can’t find it.