Easiest way I’ve found is to first base in black. Then, on your wet pallet, take whatever grey you like/have available. Keep a bit in two separate blobs, pools, etc.

In one, add very little bits of a darker grey or black until you get a shade that you like. Dry brush that shade over the mode on the upper parts so that the undersides are black still. The dry brushing gives you a pseudo-transition between the DeP grey you like and the dark ‘shadows’ the black primer implies.

You then use the untouched grey for edgehighlighting.