The face should be flat.
And the Flat of the hammer face, should strike the nipple squarely, all along the circumfrance of the nipple.
A Dremel grinding stone will fit in there nicely. Get some inletting black or lipstick and coat the top of a new nipple. Lower the hammer down to the nipple to pick up some of the transfered color, then look at the hammer face.
Grind away the color on the hammer face and repeat the proccedure until a full circle of color is transfered from the nipple to the hammer face.
This causes the hammer to fully strike the seated cap on the nipple,
,it's a great tip from Paul V.
If there is no V cut in the front of the hammer make one, the "V" helps the cap expand at firing so it's easy to remove from the cup.