Adjusting the cavesson so that it sits too low will make your horse's head look longer than it really is. The longer the head, the coarser the horse will generally look.
My rule of thumb is to adjust the cavesson noseband so that it's about two fingers width below the jaw bone. Depending on the horse's conformation, it might need to go up or down a hole from there to make it look just right.
You can compare the above photo to these next photos which have correctly adjusted cavesson nosebands, sitting right below the jaw bone:
Both heads look in proportion and complement the overall picture of the horse rather than detract from it.