Best way to cope with flat back of the head I personally think is to give some volume at the crown and make a round silhouette.
To taper the back to the crown, nape of the neck needs to be short enough.
This is my client who has flat back of the head and also fine hair.
I cut the top leaving the thickness and density but texturized enough so my client can move and volumize his hair when he style his hair by himself.
Sides and back are short. It faded up to around the crown to maximize the volume at the crown.
This is styled with hair jelly which is kind of in between gel and wax. You can style like wax adding texture, volume and movement while giving hair a shiny look.
Short sides and back make the style look fresh, clean and tidy. And adding some shape at the back and top creates nice silhouette and make the style fashionable.
He has different length of hair strands on top which adds extra dimension to his very short top.